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@clouddev1