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/requirements.md

@@ -234,9 +234,13 @@ handoff-14 cleanup; 449 after B2/C2.)
   then `ALTER TABLE` add/drop/rename column (4e — `alter` is advanced-only,
   runtime-decomposed to the existing column executors; ADD COLUMN reaches
   CREATE-TABLE constraint parity; drop/rename refuse a table-CHECK-
-  referenced column)).
-  Remaining DDL — `ALTER TABLE … ALTER COLUMN TYPE` / add-drop-constraint /
-  add-FK / `RENAME TO` (4f–4h) — is phased per ADR-0035 §13.)*
+  referenced column), then `ALTER TABLE … ALTER COLUMN TYPE` (4f —
+  runtime-decomposed to `change_column_type` with `ForceConversion`, the
+  §7 advanced policy: lossy converts with a note, incompatible + static
+  refusals (`↔ blob`, non-`int → serial`) refuse, `int → serial` allowed;
+  the internal-`__rdbms_*` guard folded into `do_change_column_type`)).
+  Remaining DDL — `ALTER TABLE` add-drop-constraint / add-FK / `RENAME TO`
+  (4g–4h) — is phased per ADR-0035 §13.)*
 - [ ] **Q2** Non-standard syntax rejected with a clear message
   pointing at the supported subset.
   *(Design done — ADR-0030 §8: out-of-subset statements are