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/dsl/grammar/mod.rs

@@ -67,6 +67,8 @@ pub enum IdentSource {
     Columns,
     /// Existing relationship name.
     Relationships,
+    /// Existing index name.
+    Indexes,
     /// Closed set from `Type::all()` — surfaced by the walker's
     /// content validator on column-type slots; not user-listable
     /// from the schema.
@@ -82,7 +84,10 @@ impl IdentSource {
     /// entities rather than user invention or a closed set).
     #[must_use]
     pub const fn completes_from_schema(self) -> bool {
-        matches!(self, Self::Tables | Self::Columns | Self::Relationships)
+        matches!(
+            self,
+            Self::Tables | Self::Columns | Self::Relationships | Self::Indexes
+        )
     }
 
     /// Human-facing label used in parse-error wording
@@ -97,6 +102,7 @@ impl IdentSource {
             Self::Tables => "table name",
             Self::Columns => "column name",
             Self::Relationships => "relationship name",
+            Self::Indexes => "index name",
             Self::Types => "type",
         }
     }
@@ -113,6 +119,7 @@ impl IdentSource {
             "table name" => Some(Self::Tables),
             "column name" => Some(Self::Columns),
             "relationship name" => Some(Self::Relationships),
+            "index name" => Some(Self::Indexes),
             "type" => Some(Self::Types),
             _ => None,
         }