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/event.rs

@@ -9,7 +9,7 @@ use crossterm::event::KeyEvent;
 
 use crate::db::{
     AddColumnResult, ChangeColumnTypeResult, DataResult, DbError, DeleteResult,
-    InsertResult, TableDescription, UpdateResult,
+    DropColumnResult, InsertResult, TableDescription, UpdateResult,
 };
 use crate::dsl::Command;
 
@@ -56,6 +56,13 @@ pub enum AppEvent {
         command: Command,
         result: AddColumnResult,
     },
+    /// A `drop column …` succeeded. `result` carries the
+    /// post-drop description plus the names of any indexes
+    /// removed by `--cascade` (ADR-0025).
+    DslDropColumnSucceeded {
+        command: Command,
+        result: DropColumnResult,
+    },
     /// A DSL command failed. `error` is the structured
     /// payload, `facts` is the runtime-built schema-resolved
     /// enrichment (parent tables, attempted values,