feat: ADR-0035 4e — ALTER TABLE add/drop/rename column

Advanced-only `alter` entry word; ALTER TABLE <T> ADD COLUMN <col> <type>
[constraints] | DROP COLUMN <col> | RENAME COLUMN <old> TO <new> ->
SqlAlterTable, runtime-decomposed to the existing column executors
(do_add_column / do_drop_column / do_rename_column) — one undo step each,
no new worker layer. The COLUMN keyword is required (reserves bare
RENAME TO for 4h, ADD CONSTRAINT for 4g).

- ADD COLUMN takes NOT NULL / UNIQUE / DEFAULT / CHECK (no PK / inline
  REFERENCES). do_add_column extended to consume the SQL raw-text
  default_sql / check_sql (sql_expr is validate-only, the 4a.2
  mechanism), reaching parity with CREATE TABLE's column constraints.
- Drop/rename column refuse a column any CHECK references — table-level
  AND column-level (incl. a column's own self-check on rename) — the
  4a.3 deferral, detected up-front by tokenizing the raw CHECK text
  (skipping string literals). In the shared executors, so it guards both
  the simple and SQL surfaces and fixes a latent rename-drift bug that
  desynced the stored CHECK text and broke rebuild.
- SQL DROP COLUMN refuses an index-covered column (no --cascade SQL
  spelling — matches SQLite + the simple default).
- The column executors and do_add_index gained an internal-__rdbms_*
  guard (refuse as "no such table"), closing a pre-existing exposure on
  both surfaces. (do_change_column_type / do_add_constraint /
  do_add_relationship are a tracked follow-up.)
- `alter` is advanced-only; AlterTableAction::AddColumn is boxed
  (clippy::large_enum_variant).

Docs: ADR-0035 status + §13 4e; ADR README; requirements.md Q1. Plan:
docs/plans/20260525-adr-0035-sql-ddl-4e.md.

Tests: 1854 passing / 0 failing / 0 skipped / 1 ignored; clippy clean.

src/dsl/grammar/mod.rs

@@ -594,6 +594,10 @@ pub static REGISTRY: &[(&CommandNode, CommandCategory)] = &[
     // `create [unique] index …` → SQL_CREATE_INDEX).
     (&ddl::SQL_CREATE_TABLE, CommandCategory::Advanced),
     (&ddl::SQL_CREATE_INDEX, CommandCategory::Advanced),
+    // `alter` is a new advanced-*only* DDL entry word (ADR-0035 §2/§4e),
+    // like `select`/`with` — no simple node, so `is_advanced_only` is
+    // true and simple-mode `alter …` gets the "this is SQL" hint.
+    (&ddl::SQL_ALTER_TABLE, CommandCategory::Advanced),
     // Shared `drop` entry word: `ddl::DROP` (simple) and these advanced
     // SQL nodes. SQL-first in advanced mode; `drop table [if exists] T`
     // → SQL_DROP_TABLE, `drop index [if exists] <name>` → SQL_DROP_INDEX