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.

src/friendly/keys.rs

@@ -198,11 +198,13 @@ pub const KEYS_AND_PLACEHOLDERS: &[(&str, &[&str])] = &[
     // code, not the catalog, because spacing is alignment-
     // sensitive in the multi-entry case.
     ("parse.usage.add_column", &[]),
+    ("parse.usage.add_index", &[]),
     ("parse.usage.add_relationship", &[]),
     ("parse.usage.change_column", &[]),
     ("parse.usage.create_table", &[]),
     ("parse.usage.delete", &[]),
     ("parse.usage.drop_column", &[]),
+    ("parse.usage.drop_index", &[]),
     ("parse.usage.drop_relationship", &[]),
     ("parse.usage.drop_table", &[]),
     ("parse.usage.insert", &[]),
@@ -394,6 +396,7 @@ pub const KEYS_AND_PLACEHOLDERS: &[(&str, &[&str])] = &[
         &["table", "column", "src_ty", "target_ty", "total"],
     ),
     // ---- DSL command success summaries (ADR-0019 §9 sweep) ----
+    ("ok.index_dropped_with_column", &["index"]),
     ("ok.rows_deleted", &["count"]),
     ("ok.rows_inserted", &["count"]),
     ("ok.rows_updated", &["count"]),

src/friendly/strings/en-US.yaml

@@ -251,13 +251,17 @@ help:
         create table <T> with pk [<col>:<type>, ...]    — create a table
     drop: |-
         drop table <T>                                  — remove a table
-        drop column [from] [table] <T>: <col>           — remove a column
+        drop column [from] [table] <T>: <col> [--cascade]   — remove a column
+          (--cascade also drops any index that covers the column)
         drop relationship <name>                        — remove a relationship
+        drop index <name>                               — remove an index
+        drop index on <T> (<col>, ...)                  — remove an index by its columns
     add: |-
         add column [to] [table] <T>: <col> (<type>)     — add a column
           (for serial/shortid on a non-empty table: existing rows auto-filled)
         add 1:n relationship [as <name>] from <P>.<col> to <C>.<col>
           [on delete <action>] [on update <action>] [--create-fk]    — declare a relationship
+        add index [as <name>] on <T> (<col>, ...)       — create an index
     rename: |-
         rename column [in] [table] <T>: <old> to <new>  — rename a column
     change: |-
@@ -412,12 +416,16 @@ parse:
     drop_relationship: |-
       drop relationship <Name>
       drop relationship from <Parent>.<col> to <Child>.<col>
+    drop_index: |-
+      drop index <Name>
+      drop index on <Table> (<col>[, ...])
     add_column: "add column [to] [table] <Table>: <Name> (<Type>)"
     add_relationship: |-
       add 1:n relationship [as <Name>]
         from <Parent>.<col> to <Child>.<col>
         [on delete <action>] [on update <action>]
         [--create-fk]
+    add_index: "add index [as <Name>] on <Table> (<col>[, ...])"
     rename_column: "rename column [in] [table] <Table>: <Old> to <New>"
     change_column: |-
       change column [in] [table] <Table>: <Name> (<Type>)
@@ -700,6 +708,9 @@ ok:
   rows_inserted: "  {count} row(s) inserted"
   rows_updated: "  {count} row(s) updated"
   rows_deleted: "  {count} row(s) deleted"
+  # Shown beneath a `drop column --cascade` summary, once per
+  # index removed because it covered the dropped column.
+  index_dropped_with_column: "  also dropped index `{index}` (it covered the column)"
 
 # ---- Client-side success notes (ADR-0017 §6, ADR-0018 §9) ------------
 client_side:

src/friendly/translate.rs

@@ -65,6 +65,8 @@ pub enum Operation {
     ChangeColumnType,
     AddRelationship,
     DropRelationship,
+    AddIndex,
+    DropIndex,
     Query,
     Rebuild,
     Replay,
@@ -92,6 +94,8 @@ impl Operation {
             Self::ChangeColumnType => "change column",
             Self::AddRelationship => "add relationship",
             Self::DropRelationship => "drop relationship",
+            Self::AddIndex => "add index",
+            Self::DropIndex => "drop index",
             Self::Query => "query",
             Self::Rebuild => "rebuild",
             Self::Replay => "replay",