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.

CLAUDE.md

@@ -186,8 +186,9 @@ not yet implemented:
 - **Column drops/renames/type changes** (B2 / C2 partial): the
   rebuild-table primitive (ADR-0013) is in place; the grammar
   and dispatch are pending.
-- **Indexes** (C3 partial): `add index`, `drop index`, then
-  `EXPLAIN QUERY PLAN` rendering for QA1.
+- **Indexes**: `add index` / `drop index` done (ADR-0025).
+  `EXPLAIN QUERY PLAN` rendering for QA1 still pending (needs
+  its own QA2 rendering ADR).
 - **Modify relationship** (C3a): drop+add covers the use case
   today.
 - **m:n convenience** (C4): auto-generates a junction table