feat: ADR-0035 4f — ALTER TABLE … ALTER COLUMN TYPE

Fourth AlterTableAction (AlterColumnType), runtime-decomposed to the
existing change_column_type executor with ForceConversion — which IS the
§7 advanced policy: lossy converts with a note (no force flag),
incompatible + the ADR-0017 static refusals (↔blob, same-type,
date↔datetime, non-int→serial) still refuse, while int→serial is allowed
(auto-fills nulls + UNIQUE, ADR-0018 §8). No new mode/note/persistence;
undo is the advanced safety net.

Grammar adds a fourth action branch leading on `alter`, discriminated in
the builder by the `type` keyword (unique — ADD COLUMN's type is an
ident); the type slot reuses SQL_TYPE. The internal-__rdbms_* guard was
folded into do_change_column_type (user-confirmed), closing the simple
`change column` exposure.

Tests: 7 Tier-3 e2e via run_replay + 4 Tier-1 parse (incl. a column-named-
`type` discriminator probe) + the simple-surface guard. Help/usage
refreshed; ADR-0035 §13 4f + README + requirements.md in lockstep.

docs/adr/0035-advanced-mode-sql-ddl.md

@@ -4,14 +4,14 @@
 
 Accepted. Design agreed with the user (2026-05-24); the approach is
 **validated end-to-end by sub-phases 4a / 4a.2 / 4a.3 / 4b / 4c / 4d /
-4e** (`CREATE TABLE` with column- and table-level constraints and foreign
-keys, `DROP TABLE [IF EXISTS]`, `CREATE [UNIQUE] INDEX` /
-`DROP INDEX [IF EXISTS]`, and `ALTER TABLE` add/drop/rename column,
-implemented 2026-05-25 — plans
+4e / 4f** (`CREATE TABLE` with column- and table-level constraints and
+foreign keys, `DROP TABLE [IF EXISTS]`, `CREATE [UNIQUE] INDEX` /
+`DROP INDEX [IF EXISTS]`, `ALTER TABLE` add/drop/rename column, and
+`ALTER TABLE … ALTER COLUMN TYPE`, implemented 2026-05-25 — plans
 `docs/plans/20260524-adr-0035-sql-ddl-4a.md`, `…-4a2.md`, `…-4a3.md`,
 `docs/plans/20260525-adr-0035-sql-ddl-4b.md`, `…-4c.md`, `…-4d.md`,
-`…-4e.md`), so the decision is accepted while the remaining sub-phases
-(**4f–4i**, §13) continue. This is **Phase 4** of the ADR-0030 roadmap (the
+`…-4e.md`, `…-4f.md`), so the decision is accepted while the remaining
+sub-phases (**4g–4i**, §13) continue. This is **Phase 4** of the ADR-0030 roadmap (the
 advanced-mode SQL surface), the peer of ADR-0031 (expression grammar),
 ADR-0032 (`SELECT`), and ADR-0033 (DML). It **clarifies ADR-0030 §4**
 on how DDL is represented and executed.
@@ -436,7 +436,25 @@ ADR-0033's structure:
   internal-table guard on `do_change_column_type` / `do_add_constraint` /
   `do_add_relationship` is a tracked follow-up.)*
 - **4f — `ALTER TABLE … ALTER COLUMN TYPE`** (the §7 conversion
-  model + the lossy-with-note path).
+  model + the lossy-with-note path). *(Implemented 2026-05-25 — plan
+  `docs/plans/20260525-adr-0035-sql-ddl-4f.md`.)* A fourth
+  `AlterTableAction::AlterColumnType`, runtime-decomposed to the existing
+  `change_column_type` executor with `ChangeColumnMode::ForceConversion`
+  — which **is** the §7 advanced policy: lossy cells are *performed* and
+  counted (the engine-neutral `client_side.transformed_lossy` note
+  fires), incompatible cells refuse, and the ADR-0017 static refusals
+  (`↔ blob`, same-type, `date ↔ datetime`, non-`int → serial`) refuse in
+  both modes. **`int → serial` is *allowed*** (auto-fills nulls, adds
+  UNIQUE if non-PK — ADR-0018 §8; the §7 "static-refused →serial"
+  summary is looser than the code). No force flag, no `USING`, no
+  `SET DATA TYPE` synonym (§7/§12); `undo` is the advanced safety net.
+  The grammar adds a fourth action branch leading on `alter`,
+  discriminated in the builder by the **`type` keyword** (unique — ADD
+  COLUMN's type is an ident); the type slot reuses `SQL_TYPE`. The
+  internal-`__rdbms_*` guard was folded into `do_change_column_type`
+  (user-confirmed 2026-05-25), closing the simple `change column`
+  exposure too. *(The remaining internal-table guard on
+  `do_add_constraint` / `do_add_relationship` rides in 4g.)*
 - **4g — `ALTER TABLE` add/drop constraint, add foreign key.**
 - **4h — `ALTER TABLE … RENAME TO`** (the §6 new low-level op).
 - **4i — Verification sweep.** Typing-surface + matrix coverage,