B2/C2: column drop / rename / change-type DSL commands

Closes B2 (rebuild-table reused outside relationships) and
C2 (full add/drop/rename/change-type column operations).

* drop column [from] [table] <T>: <col>
  - ALTER TABLE DROP COLUMN (SQLite 3.35+) + metadata
    cleanup in __rdbms_playground_columns.
  - Refuses PK columns and columns involved in a declared
    relationship (drop the relationship first).

* rename column [in] [table] <T>: <old> to <new>
  - ALTER TABLE RENAME COLUMN (SQLite 3.25+); SQLite
    cascades the rename through FK declarations on other
    tables.
  - Mirrors the new name into both metadata tables
    (__rdbms_playground_columns, __rdbms_playground_relationships)
    so describes stay accurate after a rename.
  - Refuses identity rename and name collisions.

* change column [in] [table] <T>: <col> (<newtype>)
  - Routes through the rebuild_table primitive (ADR-0013)
    since SQLite ALTER doesn't support type changes.
    INSERT INTO new SELECT FROM old; STRICT typing enforces
    cell compatibility, transaction rolls back on mismatch.
  - Refuses PK columns, relationship-involved columns,
    `serial` target, and no-op same-type changes.

Adds 20 tests (parser + db layer); updates the in-app help
listing. Both prepositions independently optional in each
new command, matching `add column`'s grammar shape.

Total: 449 passing, 0 failing, 0 skipped (up from 429).
Clippy clean.

Known spec gap: column-type-change conversion compatibility
is not yet documented (currently relies on SQLite STRICT
errors); follow-up will close this.

src/runtime.rs

@@ -1011,6 +1011,18 @@ async fn execute_command_typed(
             .add_column(table, column, ty, src)
             .await
             .map(|d| CommandOutcome::Schema(Some(d))),
+        Command::DropColumn { table, column } => database
+            .drop_column(table, column, src)
+            .await
+            .map(|d| CommandOutcome::Schema(Some(d))),
+        Command::RenameColumn { table, old, new } => database
+            .rename_column(table, old, new, src)
+            .await
+            .map(|d| CommandOutcome::Schema(Some(d))),
+        Command::ChangeColumnType { table, column, ty } => database
+            .change_column_type(table, column, ty, src)
+            .await
+            .map(|d| CommandOutcome::Schema(Some(d))),
         Command::AddRelationship {
             name,
             parent_table,