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/persistence/mod.rs

@@ -120,6 +120,11 @@ pub struct SchemaSnapshot {
     pub created_at: String,
     pub tables: Vec<TableSchema>,
     pub relationships: Vec<RelationshipSchema>,
+    /// Indexes across all tables (ADR-0025). Carried as a flat
+    /// list mirroring `relationships`; each entry names its
+    /// table. Empty for project files written before indexes
+    /// existed — the YAML field is optional on read.
+    pub indexes: Vec<IndexSchema>,
 }
 
 #[derive(Debug, Clone, PartialEq, Eq)]
@@ -142,6 +147,15 @@ pub struct ColumnSchema {
     pub unique: bool,
 }
 
+/// One index as recorded in `project.yaml` (ADR-0025).
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct IndexSchema {
+    pub name: String,
+    pub table: String,
+    /// The indexed columns, in index order.
+    pub columns: Vec<String>,
+}
+
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct RelationshipSchema {
     pub name: String,
@@ -342,6 +356,7 @@ mod tests {
             created_at: "2026-05-07T14:30:12Z".to_string(),
             tables: vec![],
             relationships: vec![],
+            indexes: vec![],
         };
         p.write_schema(&schema).unwrap();
         let body = fs::read_to_string(dir.path().join(PROJECT_YAML)).unwrap();