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/app.rs

@@ -14,7 +14,7 @@ use tracing::{trace, warn};
 use crate::action::Action;
 use crate::db::{
     AddColumnResult, CascadeEffect, ChangeColumnTypeResult, DataResult, DeleteResult,
-    InsertResult, TableDescription, UpdateResult,
+    DropColumnResult, InsertResult, TableDescription, UpdateResult,
 };
 use crate::dsl::{Command, ParseError, parse_command};
 use crate::event::AppEvent;
@@ -341,6 +341,10 @@ impl App {
                 self.handle_dsl_add_column_success(&command, result);
                 Vec::new()
             }
+            AppEvent::DslDropColumnSucceeded { command, result } => {
+                self.handle_dsl_drop_column_success(&command, result);
+                Vec::new()
+            }
             AppEvent::DslFailed {
                 command,
                 error,
@@ -1146,6 +1150,26 @@ impl App {
         self.current_table = Some(result.description);
     }
 
+    fn handle_dsl_drop_column_success(
+        &mut self,
+        command: &Command,
+        result: DropColumnResult,
+    ) {
+        self.note_ok_summary(command);
+        // ADR-0025: when `--cascade` removed covering indexes,
+        // name each one so the learner sees the side effect.
+        for index in &result.dropped_indexes {
+            self.note_system(crate::t!(
+                "ok.index_dropped_with_column",
+                index = index,
+            ));
+        }
+        for line in crate::output_render::render_structure(&result.description) {
+            self.note_system(line);
+        }
+        self.current_table = Some(result.description);
+    }
+
     fn handle_dsl_change_column_success(
         &mut self,
         command: &Command,
@@ -1250,7 +1274,7 @@ impl App {
         command: &Command,
         facts: crate::friendly::FailureContext,
     ) -> crate::friendly::TranslateContext {
-        use crate::dsl::{Command as C, RelationshipSelector};
+        use crate::dsl::{Command as C, IndexSelector, RelationshipSelector};
         use crate::friendly::{Operation, TranslateContext};
         let (operation, fallback_table, fallback_column) = match command {
             C::CreateTable { name, .. } => (Operation::CreateTable, Some(name.as_str()), None),
@@ -1260,7 +1284,7 @@ impl App {
                 Some(table.as_str()),
                 Some(column.as_str()),
             ),
-            C::DropColumn { table, column } => (
+            C::DropColumn { table, column, .. } => (
                 Operation::DropColumn,
                 Some(table.as_str()),
                 Some(column.as_str()),
@@ -1292,6 +1316,13 @@ impl App {
                 ),
                 RelationshipSelector::Named { .. } => (Operation::DropRelationship, None, None),
             },
+            C::AddIndex { table, .. } => (Operation::AddIndex, Some(table.as_str()), None),
+            C::DropIndex { selector } => match selector {
+                IndexSelector::Columns { table, .. } => {
+                    (Operation::DropIndex, Some(table.as_str()), None)
+                }
+                IndexSelector::Named { .. } => (Operation::DropIndex, None, None),
+            },
             C::Insert { table, .. } => (Operation::Insert, Some(table.as_str()), None),
             C::Update { table, .. } => (Operation::Update, Some(table.as_str()), None),
             C::Delete { table, .. } => (Operation::Delete, Some(table.as_str()), None),
@@ -2023,6 +2054,7 @@ mod tests {
             }],
             outbound_relationships: Vec::new(),
             inbound_relationships: Vec::new(),
+            indexes: Vec::new(),
         }
     }