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/runtime.rs

@@ -33,7 +33,7 @@ use crate::db::{
     Database, DbError, DeleteResult, DropColumnResult, DropIndexOutcome, DropOutcome, InsertResult,
     QueryPlan, TableDescription, UpdateResult,
 };
-use crate::dsl::{AlterTableAction, Command, ColumnSpec};
+use crate::dsl::{AlterTableAction, ChangeColumnMode, Command, ColumnSpec};
 use crate::dsl::walker::Severity;
 use crate::event::AppEvent;
 use crate::project::{
@@ -2114,6 +2114,16 @@ async fn execute_command_typed(
                 .rename_column(table, old, new, src)
                 .await
                 .map(|d| CommandOutcome::Schema(Some(d))),
+            // `ALTER COLUMN … TYPE` reuses the simple `change column`
+            // executor with `ForceConversion` — the ADR-0035 §7
+            // advanced policy (lossy converts with a note; no force
+            // flag; static-refused / incompatible still refuse). The
+            // ChangeColumn outcome surfaces the client-side lossy note,
+            // shared with simple mode.
+            AlterTableAction::AlterColumnType { column, ty } => database
+                .change_column_type(table, column, ty, ChangeColumnMode::ForceConversion, src)
+                .await
+                .map(CommandOutcome::ChangeColumn),
         },
         Command::AddConstraint {
             table,