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.

docs/requirements.md

@@ -26,12 +26,12 @@ repo is pushed).
 
 ## Test baseline
 
-After ADR-0024 full implementation + the handoff-14 cleanup
-pass: **1006 passing, 0 failing, 1 ignored** (`cargo test` —
-the one ignored test is a long-standing `` ```ignore ``
-doc-test in `src/friendly/mod.rs`). Clippy clean with the
-nursery lint group enabled. (Earlier reference point, after
-B2/C2: 449 passing.)
+After ADR-0025 (indexes): **1037 passing, 0 failing, 1
+ignored** (`cargo test` — the one ignored test is a
+long-standing `` ```ignore `` doc-test in
+`src/friendly/mod.rs`). Clippy clean with the nursery lint
+group enabled. (Earlier reference points: 1006 after ADR-0024
++ the handoff-14 cleanup; 449 after B2/C2.)
 
 ---
 
@@ -47,11 +47,12 @@ B2/C2: 449 passing.)
 
 - [ ] **S1** Three-region layout: items list (left), output
   panel (right), input field (bottom).
-- [ ] **S2** Items list shows tables and per-table indexes;
+- [x] **S2** Items list shows tables and per-table indexes;
   designed to extend to additional element kinds (relations,
   views, etc.) without restructuring.
-  *(Progress: tables are listed live from the database; indexes
-  pending alongside C3 index support.)*
+  *(ADR-0025: the items panel renders a nested list — each
+  table with its index names indented beneath it. The nested
+  model is the extension point for future element kinds.)*
 - [ ] **S3** Output panel renders a visualization of the
   currently selected item and supports multiple tabs.
 - [ ] **S4** Hint area below the input field; keyboard-toggleable
@@ -130,7 +131,9 @@ B2/C2: 449 passing.)
   FK with `ON DELETE` / `ON UPDATE` actions done (ADR-0013) —
   declared via `add 1:n relationship`; symmetric outbound +
   inbound view in the structure renderer; type compatibility
-  validated at declaration via `Type::fk_target_type()`. Index,
+  validated at declaration via `Type::fk_target_type()`.
+  Indexes done (ADR-0025) — `add index` / `drop index`,
+  rebuild-preserving, persisted in `project.yaml`.
   `NOT NULL`, `UNIQUE`, `CHECK`, `DEFAULT` still pending.)*
 - [~] **C3a** Modify relationship: `modify relationship <name>
   [on delete <action>] [on update <action>]`. Users can achieve
@@ -339,6 +342,9 @@ B2/C2: 449 passing.)
 - [ ] **QA1** `EXPLAIN QUERY PLAN` is run on demand for queries;
   output is rendered as an annotated tree highlighting full
   scans, index use, and join order.
+  *(Unblocked by ADR-0025: indexes now exist, so a plan for
+  `show data <T> where <col>=<val>` visibly changes with an
+  index. Still needs the QA2 rendering ADR.)*
 - [~] **QA2** Plan rendering specifics (tree layout, annotation
   taxonomy, colour scheme) — design and ADR pending.