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.

src/friendly/strings/en-US.yaml

@@ -273,7 +273,8 @@ help:
     sql_alter_table: |-
         alter table <T> add column <col> <type> [not null] [unique] [default …] [check …]
         alter table <T> drop column <col>
-        alter table <T> rename column <old> to <new>     — change a table's columns (advanced SQL)
+        alter table <T> rename column <old> to <new>
+        alter table <T> alter column <col> type <type>   — change a table's columns (advanced SQL)
     drop: |-
         drop table <T>                                  — remove a table
         drop column [from] [table] <T>: <col> [--cascade]   — remove a column
@@ -473,6 +474,7 @@ parse:
       alter table <Table> add column <Name> <Type> [not null] [unique] [default <expr>] [check (<expr>)]
       alter table <Table> drop column <Name>
       alter table <Table> rename column <Old> to <New>
+      alter table <Table> alter column <Name> type <Type>
     drop_table: "drop table <Name>"
     drop_column: "drop column [from] [table] <Table>: <Name>"
     drop_relationship: |-
@@ -871,8 +873,10 @@ ok:
 client_side:
   # Per-cell transformation notice when `change column ...` rewrote
   # at least one stored value (storage-class change or non-identity
-  # mapping). `lossy` variant fires under --force-conversion when
-  # information was discarded.
+  # mapping). `lossy` variant fires when information was discarded —
+  # under simple-mode `--force-conversion`, and under advanced-mode
+  # `alter table … alter column … type …`, which always converts
+  # (ADR-0035 §7).
   transformed: |-
     [client-side] {count} row(s) were transformed before being stored. In raw SQL this would need an explicit `CAST` or application-level code.
   transformed_lossy: |-