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/ui.rs

@@ -420,20 +420,27 @@ fn render_items_panel(app: &App, theme: &Theme, frame: &mut Frame<'_>, area: Rec
         .as_ref()
         .map(|t| t.name.as_str())
         .unwrap_or_default();
-    let lines: Vec<Line<'_>> = app
-        .tables
-        .iter()
-        .map(|name| {
-            let style = if name == highlight {
-                Style::default()
-                    .fg(theme.fg)
-                    .add_modifier(Modifier::BOLD)
-            } else {
-                Style::default().fg(theme.fg)
-            };
-            Line::from(Span::styled(name.as_str(), style))
-        })
-        .collect();
+    // Nested tables / per-table indexes (S2, ADR-0025): each
+    // table line, with its index names indented beneath it.
+    let mut lines: Vec<Line<'_>> = Vec::new();
+    for name in &app.tables {
+        let style = if name == highlight {
+            Style::default()
+                .fg(theme.fg)
+                .add_modifier(Modifier::BOLD)
+        } else {
+            Style::default().fg(theme.fg)
+        };
+        lines.push(Line::from(Span::styled(name.as_str(), style)));
+        if let Some(indexes) = app.schema_cache.table_indexes.get(name) {
+            for index in indexes {
+                lines.push(Line::from(Span::styled(
+                    format!("  {index}"),
+                    Style::default().fg(theme.muted),
+                )));
+            }
+        }
+    }
     let paragraph = Paragraph::new(lines).block(block);
     frame.render_widget(paragraph, area);
 }
@@ -1013,6 +1020,7 @@ mod tests {
             ],
             outbound_relationships: Vec::new(),
             inbound_relationships: Vec::new(),
+            indexes: Vec::new(),
         };
         app.current_table = Some(desc);
         // Mirror what the App writes when a DSL command succeeds.
@@ -1041,4 +1049,21 @@ mod tests {
         let snapshot = render_to_string(&mut app, &theme, 80, 24);
         insta::assert_snapshot!("populated_with_table_dark", snapshot);
     }
+
+    #[test]
+    fn items_panel_nests_indexes_under_their_table() {
+        // S2 (ADR-0025): the items panel renders each table
+        // with its index names indented beneath it.
+        let mut app = App::new();
+        app.tables = vec!["Customers".to_string(), "Orders".to_string()];
+        app.schema_cache.table_indexes.insert(
+            "Customers".to_string(),
+            vec!["idx_email".to_string()],
+        );
+        let theme = Theme::dark();
+        let out = render_to_string(&mut app, &theme, 80, 24);
+        assert!(out.contains("Customers"), "table listed:\n{out}");
+        assert!(out.contains("Orders"), "table listed:\n{out}");
+        assert!(out.contains("idx_email"), "index nested in panel:\n{out}");
+    }
 }