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/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",