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/walking_skeleton.rs

@@ -257,6 +257,7 @@ fn fake_table(name: &str, columns: &[(&str, Type, bool)]) -> TableDescription {
             .collect(),
         outbound_relationships: Vec::new(),
         inbound_relationships: Vec::new(),
+        indexes: Vec::new(),
     }
 }
 
@@ -422,6 +423,7 @@ fn add_relationship_flow_shows_parent_side_with_inbound_section() {
             on_delete: ReferentialAction::Cascade,
             on_update: ReferentialAction::NoAction,
         }],
+        indexes: Vec::new(),
     };
     app.update(AppEvent::DslSucceeded {
         command: Command::AddRelationship {
@@ -470,6 +472,7 @@ fn add_relationship_flow_shows_inbound_section_on_parent() {
             on_delete: ReferentialAction::Cascade,
             on_update: ReferentialAction::NoAction,
         }],
+        indexes: Vec::new(),
     };
     app.update(AppEvent::DslSucceeded {
         command: Command::AddColumn {