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/dsl/mod.rs

@@ -20,8 +20,8 @@ pub mod walker;
 
 pub use action::ReferentialAction;
 pub use command::{
-    AppCommand, ChangeColumnMode, ColumnSpec, Command, MessagesValue, ModeValue,
-    RelationshipSelector, RowFilter,
+    AppCommand, ChangeColumnMode, ColumnSpec, Command, IndexSelector, MessagesValue,
+    ModeValue, RelationshipSelector, RowFilter,
 };
 pub use parser::{ParseError, parse_command};
 pub use types::Type;