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/keys.rs

@@ -198,11 +198,13 @@ pub const KEYS_AND_PLACEHOLDERS: &[(&str, &[&str])] = &[
     // code, not the catalog, because spacing is alignment-
     // sensitive in the multi-entry case.
     ("parse.usage.add_column", &[]),
+    ("parse.usage.add_index", &[]),
     ("parse.usage.add_relationship", &[]),
     ("parse.usage.change_column", &[]),
     ("parse.usage.create_table", &[]),
     ("parse.usage.delete", &[]),
     ("parse.usage.drop_column", &[]),
+    ("parse.usage.drop_index", &[]),
     ("parse.usage.drop_relationship", &[]),
     ("parse.usage.drop_table", &[]),
     ("parse.usage.insert", &[]),
@@ -394,6 +396,7 @@ pub const KEYS_AND_PLACEHOLDERS: &[(&str, &[&str])] = &[
         &["table", "column", "src_ty", "target_ty", "total"],
     ),
     // ---- DSL command success summaries (ADR-0019 §9 sweep) ----
+    ("ok.index_dropped_with_column", &["index"]),
     ("ok.rows_deleted", &["count"]),
     ("ok.rows_inserted", &["count"]),
     ("ok.rows_updated", &["count"]),