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);
+}