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.

docs/requirements.md

@@ -26,10 +26,9 @@ repo is pushed).
 
 ## Test baseline
 
-After Iterations 5 + 6 (export/import + --resume + persistent
-input history + migration scaffold): **408 passing, 0 failing,
-0 skipped** (`cargo test`). Clippy clean with the nursery
-lint group enabled.
+After B2/C2 (column drop / rename / change-type): **449
+passing, 0 failing, 0 skipped** (`cargo test`). Clippy clean
+with the nursery lint group enabled.
 
 ---
 
@@ -112,11 +111,14 @@ lint group enabled.
 
 - [ ] **C1** Table operations: create / drop / rename.
   *(Progress: create + drop done; rename pending.)*
-- [ ] **C2** Column operations: add / drop / rename / change
-  type, including the rebuild-table dance behind the scenes
-  where SQLite ALTER cannot do it directly.
-  *(Progress: add done; drop/rename/change-type pending — the
-  rebuild-table dance is the gating piece, B2.)*
+- [x] **C2** Column operations: add / drop / rename / change
+  type. `drop column` and `rename column` use SQLite native
+  ALTER TABLE (3.35+ / 3.25+); `change column` routes through
+  the rebuild-table primitive since ALTER doesn't support
+  type changes. PK and relationship-involved columns are
+  refused with friendly messages (drop the relationship
+  first); SQLite STRICT enforces type compatibility on the
+  data copy during a type change.
 - [ ] **C3** Schema constraints: primary key (single and
   compound), foreign key with `ON DELETE` / `ON UPDATE` referential
   actions, indexes, `NOT NULL`, `UNIQUE`, `CHECK`, `DEFAULT`.
@@ -166,12 +168,14 @@ lint group enabled.
 - [x] **B1** SQLite via `rusqlite`; all tables created `STRICT`;
   `PRAGMA foreign_keys = ON` per connection. *(Database accessed
   through a dedicated worker thread per ADR-0010.)*
-- [ ] **B2** Schema evolution uses the rebuild-table technique
-  internally where SQLite `ALTER TABLE` cannot.
-  *(Progress: rebuild-table primitive landed (ADR-0013) and is
-  used by `add_relationship` / `drop_relationship`. Reuse for
-  column drops/renames/type changes pending; the primitive is
-  designed to support those without further architectural work.)*
+- [x] **B2** Schema evolution uses the rebuild-table technique
+  internally where SQLite `ALTER TABLE` cannot — currently
+  the change-column-type code path. Add-column, drop-column,
+  and rename-column take the simpler ALTER TABLE route since
+  modern SQLite supports them natively; metadata sync into
+  `__rdbms_playground_columns` and
+  `__rdbms_playground_relationships` happens in the same
+  transaction either way.
 - [ ] **B3** Query timeout and cancellation supported (no
   cartesian-join-of-doom can hang the app).
   *(Progress: the worker-thread architecture is in place; the