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.

tests/typing_surface/mod.rs

@@ -32,6 +32,7 @@ pub mod create_table;
 pub mod drop_column;
 pub mod drop_relationship;
 pub mod add_relationship;
+pub mod index_ops;
 pub mod rename_change_column;
 pub mod app_commands;
 pub mod candidate_ordering;
@@ -203,6 +204,8 @@ fn command_kind_label(cmd: &rdbms_playground::dsl::Command) -> String {
         ChangeColumnType { .. } => "ChangeColumnType".into(),
         AddRelationship { .. } => "AddRelationship".into(),
         DropRelationship { .. } => "DropRelationship".into(),
+        AddIndex { .. } => "AddIndex".into(),
+        DropIndex { .. } => "DropIndex".into(),
         ShowTable { .. } => "ShowTable".into(),
         Insert { .. } => "Insert".into(),
         Update { .. } => "Update".into(),