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/app.rs

@@ -1614,6 +1614,21 @@ impl App {
                     Some(table.as_str()),
                     Some(column.as_str()),
                 ),
+                // ADR-0035 Amendment 2: the column-attribute set/drop forms
+                // decompose to add/drop constraint, and name a column (so
+                // the friendly error can pinpoint it).
+                AlterTableAction::SetColumnNotNull { column }
+                | AlterTableAction::SetColumnDefault { column, .. } => (
+                    Operation::AddConstraint,
+                    Some(table.as_str()),
+                    Some(column.as_str()),
+                ),
+                AlterTableAction::DropColumnNotNull { column }
+                | AlterTableAction::DropColumnDefault { column } => (
+                    Operation::DropConstraint,
+                    Some(table.as_str()),
+                    Some(column.as_str()),
+                ),
                 AlterTableAction::AddTableConstraint { .. } => {
                     (Operation::AddConstraint, Some(table.as_str()), None)
                 }