Indexes: add index / drop index, persistence, display (ADR-0025)

Implement ADR-0025 — indexes as a DSL DDL feature.

- Grammar: `add index [as <name>] on <T> (<cols>)`, `drop index
  <name>` / `drop index on <T> (<cols>)`, plus a `--cascade`
  flag on `drop column`.
- db.rs: index operations over the engine's native index
  catalog (no metadata table). The rebuild-table primitive now
  captures and recreates indexes, so `change column` and the
  relationship operations no longer silently drop them.
- `drop column` refuses an indexed column unless `--cascade`,
  which drops the covering indexes and reports each.
- Persistence: additive `indexes:` list in `project.yaml`
  (version unchanged); round-trips through rebuild/export/import.
- Display: an `Indexes:` section in the structure view and a
  nested tables/indexes items panel (S2).

Reconciles requirements.md (C3 index portion, S2 satisfied)
and CLAUDE.md. 1038 tests passing (+31), clippy clean.

tests/typing_surface/index_ops.rs

@@ -0,0 +1,78 @@
+//! Matrix coverage for `add index [as <name>] on <T> (<col>, …)`
+//! and `drop index (<name> | on <T> (<col>, …))` (ADR-0025).
+
+use crate::typing_surface::*;
+use rdbms_playground::input_render::InputState;
+
+#[test]
+fn after_add_offers_index_branch() {
+    let schema = schema_multi_table();
+    let a = assess_at_end("add ", &schema);
+    assert!(matches!(a.state, InputState::IncompleteAtEof));
+    assert_candidate_present(&a, &["index"]);
+    crate::snap!("after_add_index_branch", a);
+}
+
+#[test]
+fn add_index_after_on_offers_table_names() {
+    let schema = schema_multi_table();
+    let a = assess_at_end("add index on ", &schema);
+    assert!(matches!(a.state, InputState::IncompleteAtEof));
+    assert_candidate_present(&a, &["Customers", "Orders"]);
+    crate::snap!("add_index_after_on", a);
+}
+
+#[test]
+fn add_index_open_paren_narrows_to_table_columns() {
+    let schema = schema_multi_table();
+    let a = assess_at_end("add index on Orders (", &schema);
+    assert!(matches!(a.state, InputState::IncompleteAtEof));
+    assert_candidate_present(&a, &["OrderId", "CustId", "Total"]);
+    assert_no_candidate_named(&a, &["id", "Name"]);
+    crate::snap!("add_index_open_paren", a);
+}
+
+#[test]
+fn complete_add_index_parses() {
+    let schema = schema_multi_table();
+    let a = assess_at_end("add index on Orders (CustId)", &schema);
+    assert!(matches!(a.state, InputState::Valid));
+    assert_eq!(a.parse_result.as_deref(), Ok("AddIndex"));
+    crate::snap!("add_index_complete", a);
+}
+
+#[test]
+fn complete_add_index_named_parses() {
+    let schema = schema_multi_table();
+    let a = assess_at_end("add index as ord_cust on Orders (CustId)", &schema);
+    assert!(matches!(a.state, InputState::Valid));
+    assert_eq!(a.parse_result.as_deref(), Ok("AddIndex"));
+    crate::snap!("add_index_named_complete", a);
+}
+
+#[test]
+fn after_drop_offers_index_branch() {
+    let schema = schema_multi_table();
+    let a = assess_at_end("drop ", &schema);
+    assert!(matches!(a.state, InputState::IncompleteAtEof));
+    assert_candidate_present(&a, &["index"]);
+    crate::snap!("drop_index_branch", a);
+}
+
+#[test]
+fn complete_drop_index_by_name_parses() {
+    let schema = schema_multi_table();
+    let a = assess_at_end("drop index some_idx", &schema);
+    assert!(matches!(a.state, InputState::Valid));
+    assert_eq!(a.parse_result.as_deref(), Ok("DropIndex"));
+    crate::snap!("drop_index_named_complete", a);
+}
+
+#[test]
+fn complete_drop_index_by_columns_parses() {
+    let schema = schema_multi_table();
+    let a = assess_at_end("drop index on Orders (CustId)", &schema);
+    assert!(matches!(a.state, InputState::Valid));
+    assert_eq!(a.parse_result.as_deref(), Ok("DropIndex"));
+    crate::snap!("drop_index_columns_complete", a);
+}

tests/typing_surface/mod.rs

@@ -32,6 +32,7 @@ pub mod create_table;
 pub mod drop_column;
 pub mod drop_relationship;
 pub mod add_relationship;
+pub mod index_ops;
 pub mod rename_change_column;
 pub mod app_commands;
 pub mod candidate_ordering;
@@ -203,6 +204,8 @@ fn command_kind_label(cmd: &rdbms_playground::dsl::Command) -> String {
         ChangeColumnType { .. } => "ChangeColumnType".into(),
         AddRelationship { .. } => "AddRelationship".into(),
         DropRelationship { .. } => "DropRelationship".into(),
+        AddIndex { .. } => "AddIndex".into(),
+        DropIndex { .. } => "DropIndex".into(),
         ShowTable { .. } => "ShowTable".into(),
         Insert { .. } => "Insert".into(),
         Update { .. } => "Update".into(),

tests/typing_surface/snapshots/typing_surface_matrix__typing_surface__add_relationship__after_add_offers_relationship_branch@after_add.snap

@@ -14,6 +14,10 @@ Assessment {
                     text: "column",
                     kind: Keyword,
                 },
+                Candidate {
+                    text: "index",
+                    kind: Keyword,
+                },
                 Candidate {
                     text: "1:n",
                     kind: Keyword,
@@ -34,6 +38,10 @@ Assessment {
                     text: "column",
                     kind: Keyword,
                 },
+                Candidate {
+                    text: "index",
+                    kind: Keyword,
+                },
                 Candidate {
                     text: "1:n",
                     kind: Keyword,

tests/typing_surface/snapshots/typing_surface_matrix__typing_surface__index_ops__add_index_after_on_offers_table_names@add_index_after_on.snap

@@ -0,0 +1,47 @@
+---
+source: tests/typing_surface/index_ops.rs
+description: "input=\"add index on \" cursor=13"
+expression: "& a"
+---
+Assessment {
+    input: "add index on ",
+    cursor: 13,
+    state: IncompleteAtEof,
+    hint: Some(
+        Candidates {
+            items: [
+                Candidate {
+                    text: "Customers",
+                    kind: Identifier,
+                },
+                Candidate {
+                    text: "Orders",
+                    kind: Identifier,
+                },
+            ],
+            selected: None,
+        },
+    ),
+    completion: Some(
+        Completion {
+            replaced_range: (
+                13,
+                13,
+            ),
+            partial_prefix: "",
+            candidates: [
+                Candidate {
+                    text: "Customers",
+                    kind: Identifier,
+                },
+                Candidate {
+                    text: "Orders",
+                    kind: Identifier,
+                },
+            ],
+        },
+    ),
+    parse_result: Err(
+        "Invalid(at_eof)",
+    ),
+}

tests/typing_surface/snapshots/typing_surface_matrix__typing_surface__index_ops__add_index_open_paren_narrows_to_table_columns@add_index_open_paren.snap

@@ -0,0 +1,55 @@
+---
+source: tests/typing_surface/index_ops.rs
+description: "input=\"add index on Orders (\" cursor=21"
+expression: "& a"
+---
+Assessment {
+    input: "add index on Orders (",
+    cursor: 21,
+    state: IncompleteAtEof,
+    hint: Some(
+        Candidates {
+            items: [
+                Candidate {
+                    text: "CustId",
+                    kind: Identifier,
+                },
+                Candidate {
+                    text: "OrderId",
+                    kind: Identifier,
+                },
+                Candidate {
+                    text: "Total",
+                    kind: Identifier,
+                },
+            ],
+            selected: None,
+        },
+    ),
+    completion: Some(
+        Completion {
+            replaced_range: (
+                21,
+                21,
+            ),
+            partial_prefix: "",
+            candidates: [
+                Candidate {
+                    text: "CustId",
+                    kind: Identifier,
+                },
+                Candidate {
+                    text: "OrderId",
+                    kind: Identifier,
+                },
+                Candidate {
+                    text: "Total",
+                    kind: Identifier,
+                },
+            ],
+        },
+    ),
+    parse_result: Err(
+        "Invalid(at_eof)",
+    ),
+}

tests/typing_surface/snapshots/typing_surface_matrix__typing_surface__index_ops__after_add_offers_index_branch@after_add_index_branch.snap

@@ -0,0 +1,55 @@
+---
+source: tests/typing_surface/index_ops.rs
+description: "input=\"add \" cursor=4"
+expression: "& a"
+---
+Assessment {
+    input: "add ",
+    cursor: 4,
+    state: IncompleteAtEof,
+    hint: Some(
+        Candidates {
+            items: [
+                Candidate {
+                    text: "column",
+                    kind: Keyword,
+                },
+                Candidate {
+                    text: "index",
+                    kind: Keyword,
+                },
+                Candidate {
+                    text: "1:n",
+                    kind: Keyword,
+                },
+            ],
+            selected: None,
+        },
+    ),
+    completion: Some(
+        Completion {
+            replaced_range: (
+                4,
+                4,
+            ),
+            partial_prefix: "",
+            candidates: [
+                Candidate {
+                    text: "column",
+                    kind: Keyword,
+                },
+                Candidate {
+                    text: "index",
+                    kind: Keyword,
+                },
+                Candidate {
+                    text: "1:n",
+                    kind: Keyword,
+                },
+            ],
+        },
+    ),
+    parse_result: Err(
+        "Invalid(at_eof)",
+    ),
+}

tests/typing_surface/snapshots/typing_surface_matrix__typing_surface__index_ops__after_drop_offers_index_branch@drop_index_branch.snap

@@ -0,0 +1,63 @@
+---
+source: tests/typing_surface/index_ops.rs
+description: "input=\"drop \" cursor=5"
+expression: "& a"
+---
+Assessment {
+    input: "drop ",
+    cursor: 5,
+    state: IncompleteAtEof,
+    hint: Some(
+        Candidates {
+            items: [
+                Candidate {
+                    text: "column",
+                    kind: Keyword,
+                },
+                Candidate {
+                    text: "relationship",
+                    kind: Keyword,
+                },
+                Candidate {
+                    text: "table",
+                    kind: Keyword,
+                },
+                Candidate {
+                    text: "index",
+                    kind: Keyword,
+                },
+            ],
+            selected: None,
+        },
+    ),
+    completion: Some(
+        Completion {
+            replaced_range: (
+                5,
+                5,
+            ),
+            partial_prefix: "",
+            candidates: [
+                Candidate {
+                    text: "column",
+                    kind: Keyword,
+                },
+                Candidate {
+                    text: "relationship",
+                    kind: Keyword,
+                },
+                Candidate {
+                    text: "table",
+                    kind: Keyword,
+                },
+                Candidate {
+                    text: "index",
+                    kind: Keyword,
+                },
+            ],
+        },
+    ),
+    parse_result: Err(
+        "Invalid(at_eof)",
+    ),
+}

tests/typing_surface/snapshots/typing_surface_matrix__typing_surface__index_ops__complete_add_index_named_parses@add_index_named_complete.snap

@@ -0,0 +1,19 @@
+---
+source: tests/typing_surface/index_ops.rs
+description: "input=\"add index as ord_cust on Orders (CustId)\" cursor=40"
+expression: "& a"
+---
+Assessment {
+    input: "add index as ord_cust on Orders (CustId)",
+    cursor: 40,
+    state: Valid,
+    hint: Some(
+        Prose(
+            "Submit with Enter",
+        ),
+    ),
+    completion: None,
+    parse_result: Ok(
+        "AddIndex",
+    ),
+}

tests/typing_surface/snapshots/typing_surface_matrix__typing_surface__index_ops__complete_add_index_parses@add_index_complete.snap

@@ -0,0 +1,19 @@
+---
+source: tests/typing_surface/index_ops.rs
+description: "input=\"add index on Orders (CustId)\" cursor=28"
+expression: "& a"
+---
+Assessment {
+    input: "add index on Orders (CustId)",
+    cursor: 28,
+    state: Valid,
+    hint: Some(
+        Prose(
+            "Submit with Enter",
+        ),
+    ),
+    completion: None,
+    parse_result: Ok(
+        "AddIndex",
+    ),
+}

tests/typing_surface/snapshots/typing_surface_matrix__typing_surface__index_ops__complete_drop_index_by_columns_parses@drop_index_columns_complete.snap

@@ -0,0 +1,19 @@
+---
+source: tests/typing_surface/index_ops.rs
+description: "input=\"drop index on Orders (CustId)\" cursor=29"
+expression: "& a"
+---
+Assessment {
+    input: "drop index on Orders (CustId)",
+    cursor: 29,
+    state: Valid,
+    hint: Some(
+        Prose(
+            "Submit with Enter",
+        ),
+    ),
+    completion: None,
+    parse_result: Ok(
+        "DropIndex",
+    ),
+}

tests/typing_surface/snapshots/typing_surface_matrix__typing_surface__index_ops__complete_drop_index_by_name_parses@drop_index_named_complete.snap

@@ -0,0 +1,19 @@
+---
+source: tests/typing_surface/index_ops.rs
+description: "input=\"drop index some_idx\" cursor=19"
+expression: "& a"
+---
+Assessment {
+    input: "drop index some_idx",
+    cursor: 19,
+    state: Valid,
+    hint: Some(
+        Prose(
+            "No such identifier: `some_idx`",
+        ),
+    ),
+    completion: None,
+    parse_result: Ok(
+        "DropIndex",
+    ),
+}