feat: advanced ALTER COLUMN SET/DROP NOT NULL & DEFAULT, SET DATA TYPE (ADR-0035 Am2)

The standard-first ALTER COLUMN constraint gap-fill advanced mode lacked:

- ALTER COLUMN <c> SET DATA TYPE <ty> — ISO canonical synonym for the
  PostgreSQL TYPE shorthand (same AlterColumnType action + executor).
- SET NOT NULL / DROP NOT NULL — reuse the ADR-0029 do_add_constraint /
  do_drop_constraint executors (dry-run + internal-table guards free).
- SET DEFAULT <expr> / DROP DEFAULT — SET DEFAULT uses a dedicated
  raw-SQL executor (do_set_column_default); sql_expr yields no typed
  Value, so it can't go through do_add_constraint. DROP DEFAULT reuses
  do_drop_constraint.

Grammar: AT_ALTER_COLUMN gains a tail Choice (type / set / drop), reusing
SQL_TYPE and the CREATE TABLE DEFAULT_NODES; builder dispatch routes the
new column-attribute forms; runtime decomposes to the executors.

ADR-0035 Am2 corrected in-place: SET DEFAULT decomposes to
do_set_column_default, not do_add_constraint (Value-based) — found during
build.

Tests (test-first): 6 parse + 7 Tier-3 execution via run_replay. Suite
1962/0/1; clippy clean.

src/dsl/command.rs

@@ -756,8 +756,26 @@ pub enum AlterTableAction {
     /// with `ChangeColumnMode::ForceConversion`, which is the ADR-0035 §7
     /// advanced-mode policy (lossy cells are *performed* with a note, no
     /// force flag; static-refused / incompatible still refuse). One undo
-    /// step (the executor's rebuild). ADR-0035 §4f.
+    /// step (the executor's rebuild). ADR-0035 §4f. The ISO synonym
+    /// `SET DATA TYPE` (canonical) and the PostgreSQL `TYPE` shorthand
+    /// both build this action (ADR-0035 Amendment 2).
     AlterColumnType { column: String, ty: Type },
+    /// `ALTER COLUMN <name> SET NOT NULL` (ADR-0035 Amendment 2) — a
+    /// documented PostgreSQL extension (ISO has no in-place NOT-NULL
+    /// verb). Decomposes to `do_add_constraint(NotNull)` (ADR-0029).
+    SetColumnNotNull { column: String },
+    /// `ALTER COLUMN <name> DROP NOT NULL` (ADR-0035 Amendment 2).
+    /// Decomposes to `do_drop_constraint(NotNull)`.
+    DropColumnNotNull { column: String },
+    /// `ALTER COLUMN <name> SET DEFAULT <expr>` (ADR-0035 Amendment 2,
+    /// ISO). `default_sql` is the raw `sql_expr` text (the §4a.2 / §4e
+    /// mechanism — `sql_expr` builds no AST, so the default cannot be a
+    /// typed `Value`). Decomposes to the dedicated `do_set_column_default`
+    /// executor (not `do_add_constraint`, which is `Value`-based).
+    SetColumnDefault { column: String, default_sql: String },
+    /// `ALTER COLUMN <name> DROP DEFAULT` (ADR-0035 Amendment 2, ISO).
+    /// Decomposes to `do_drop_constraint(Default)`.
+    DropColumnDefault { column: String },
     /// `ADD [CONSTRAINT <name>] (CHECK (…) | UNIQUE (…) | FOREIGN KEY
     /// (…) REFERENCES …)` — a table-level constraint (ADR-0035 §4g). The
     /// `name` is the `CONSTRAINT <name>` prefix (the FK carries its own