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/output_render.rs

@@ -132,6 +132,19 @@ pub fn render_structure(desc: &TableDescription) -> Vec<String> {
         }
     }
 
+    // Indexes section (ADR-0025), shown only when the table
+    // carries at least one user-created index.
+    if !desc.indexes.is_empty() {
+        out.push("Indexes:".to_string());
+        for index in &desc.indexes {
+            out.push(format!(
+                "  {} ({})",
+                index.name,
+                index.columns.join(", "),
+            ));
+        }
+    }
+
     out
 }
 
@@ -331,7 +344,7 @@ fn content_row(cells: &[String], widths: &[usize], alignments: &[Alignment]) ->
 #[cfg(test)]
 mod tests {
     use super::*;
-    use crate::db::{ColumnDescription, RelationshipEnd};
+    use crate::db::{ColumnDescription, IndexInfo, RelationshipEnd};
     use crate::dsl::ReferentialAction;
     use insta::assert_snapshot;
 
@@ -548,6 +561,7 @@ mod tests {
             ],
             outbound_relationships: Vec::new(),
             inbound_relationships: Vec::new(),
+            indexes: Vec::new(),
         };
         assert_snapshot!(render_structure(&desc).join("\n"));
     }
@@ -566,6 +580,7 @@ mod tests {
                 on_delete: ReferentialAction::Cascade,
                 on_update: ReferentialAction::NoAction,
             }],
+            indexes: Vec::new(),
         };
         let out = render_structure(&desc).join("\n");
         assert!(
@@ -590,6 +605,7 @@ mod tests {
             ],
             outbound_relationships: Vec::new(),
             inbound_relationships: Vec::new(),
+            indexes: Vec::new(),
         };
         let out = render_structure(&desc).join("\n");
         // PK appears for id, NOT NULL for name, blank for nick.
@@ -597,6 +613,40 @@ mod tests {
         assert!(out.contains("│ name │ text   │ NOT NULL"), "got:\n{out}");
     }
 
+    #[test]
+    fn render_structure_shows_indexes_section() {
+        let desc = TableDescription {
+            name: "Customers".to_string(),
+            columns: vec![
+                col("id", Type::Serial, true, false),
+                col("Email", Type::Text, false, false),
+            ],
+            outbound_relationships: Vec::new(),
+            inbound_relationships: Vec::new(),
+            indexes: vec![IndexInfo {
+                name: "idx_email".to_string(),
+                columns: vec!["Email".to_string()],
+                unique: false,
+            }],
+        };
+        let out = render_structure(&desc).join("\n");
+        assert!(out.contains("Indexes:"), "got:\n{out}");
+        assert!(out.contains("idx_email (Email)"), "got:\n{out}");
+    }
+
+    #[test]
+    fn render_structure_omits_indexes_section_when_none() {
+        let desc = TableDescription {
+            name: "T".to_string(),
+            columns: vec![col("id", Type::Serial, true, false)],
+            outbound_relationships: Vec::new(),
+            inbound_relationships: Vec::new(),
+            indexes: Vec::new(),
+        };
+        let out = render_structure(&desc).join("\n");
+        assert!(!out.contains("Indexes:"), "got:\n{out}");
+    }
+
     #[test]
     fn render_structure_falls_back_to_sqlite_type_when_user_type_missing() {
         let mut desc = TableDescription {
@@ -610,6 +660,7 @@ mod tests {
             }],
             outbound_relationships: Vec::new(),
             inbound_relationships: Vec::new(),
+            indexes: Vec::new(),
         };
         let out = render_structure(&desc).join("\n");
         // The lowercase form of the SQLite type should appear.