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/runtime.rs

@@ -33,7 +33,7 @@ use crate::db::{
     Database, DbError, DeleteResult, DropColumnResult, DropIndexOutcome, DropOutcome, InsertResult,
     QueryPlan, TableDescription, UpdateResult,
 };
-use crate::dsl::command::TableConstraint;
+use crate::dsl::command::{Constraint, ConstraintKind, TableConstraint};
 use crate::dsl::{AlterTableAction, ChangeColumnMode, Command, ColumnSpec};
 use crate::dsl::walker::Severity;
 use crate::event::AppEvent;
@@ -2185,6 +2185,26 @@ async fn execute_command_typed(
                 .change_column_type(table, column, ty, ChangeColumnMode::ForceConversion, src)
                 .await
                 .map(CommandOutcome::ChangeColumn),
+            // ADR-0035 Amendment 2: ALTER COLUMN constraint gap-fill.
+            // SET/DROP NOT NULL and DROP DEFAULT reuse the ADR-0029
+            // executors; SET DEFAULT needs the raw-SQL path (sql_expr has
+            // no typed Value).
+            AlterTableAction::SetColumnNotNull { column } => database
+                .add_constraint(table, column, Constraint::NotNull, src)
+                .await
+                .map(|d| CommandOutcome::Schema(Some(d))),
+            AlterTableAction::DropColumnNotNull { column } => database
+                .drop_constraint(table, column, ConstraintKind::NotNull, src)
+                .await
+                .map(|d| CommandOutcome::Schema(Some(d))),
+            AlterTableAction::SetColumnDefault { column, default_sql } => database
+                .set_column_default(table, column, default_sql, src)
+                .await
+                .map(|d| CommandOutcome::Schema(Some(d))),
+            AlterTableAction::DropColumnDefault { column } => database
+                .drop_constraint(table, column, ConstraintKind::Default, src)
+                .await
+                .map(|d| CommandOutcome::Schema(Some(d))),
             // `ADD [CONSTRAINT <name>] (CHECK | UNIQUE | FOREIGN KEY)`
             // (ADR-0035 §4g) — each reuses an existing low-level executor
             // (the FK via the relationship machinery `add 1:n