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/command.rs

@@ -301,6 +301,15 @@ pub enum Command {
         unique: bool,
         if_not_exists: bool,
     },
+    /// Advanced-mode SQL `ALTER TABLE <table> <action>` (ADR-0035 §4,
+    /// sub-phase 4e). `alter` is advanced-only. Each action maps to an
+    /// existing column executor — the runtime decomposes it to
+    /// `add_column` / `drop_column` / `rename_column` (one undo step
+    /// each). 4f/4g/4h extend [`AlterTableAction`].
+    SqlAlterTable {
+        table: String,
+        action: AlterTableAction,
+    },
     /// Add a column-level constraint to an existing column
     /// (ADR-0029 §2.2). Applied through the rebuild-table
     /// primitive after a §5 dry-run guards populated columns.
@@ -705,6 +714,25 @@ pub enum IndexSelector {
     Columns { table: String, columns: Vec<String> },
 }
 
+/// The action of an advanced-mode `ALTER TABLE` (ADR-0035 §4). Sub-phase
+/// 4e carries the column actions; 4f/4g/4h add `AlterColumnType`,
+/// `AddConstraint`/`AddForeignKey`/`DropConstraint`, and `RenameTo`.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum AlterTableAction {
+    /// `ADD COLUMN <name> <type> [NOT NULL] [UNIQUE] [DEFAULT …]
+    /// [CHECK …]` — column constraints only (no PK / inline REFERENCES;
+    /// those are create-table / 4g). Reuses `do_add_column`. Boxed so
+    /// the large `ColumnSpec` doesn't bloat the enum (and `Command` /
+    /// `Action` that embed it) — `clippy::large_enum_variant`.
+    AddColumn(Box<ColumnSpec>),
+    /// `DROP COLUMN <name>` — reuses `do_drop_column` (cascade = false:
+    /// an index-covered column is refused, matching SQLite + the
+    /// simple-mode default; there is no `--cascade` SQL spelling).
+    DropColumn { column: String },
+    /// `RENAME COLUMN <old> TO <new>` — reuses `do_rename_column`.
+    RenameColumn { old: String, new: String },
+}
+
 impl std::fmt::Display for IndexSelector {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         match self {
@@ -735,6 +763,7 @@ impl Command {
             Self::DropIndex { .. } => "drop index",
             Self::SqlDropIndex { .. } => "drop index",
             Self::SqlCreateIndex { .. } => "create index",
+            Self::SqlAlterTable { .. } => "alter table",
             Self::AddConstraint { .. } => "add constraint",
             Self::DropConstraint { .. } => "drop constraint",
             Self::ShowTable { .. } => "show table",
@@ -813,6 +842,7 @@ impl Command {
             // `DropIndex` / `SqlDropTable` fallback).
             Self::SqlDropIndex { name, .. } => name,
             Self::SqlCreateIndex { table, .. } => table,
+            Self::SqlAlterTable { table, .. } => table,
             // Replay isn't tied to a single table; the path is
             // the most identifying thing for log output.
             Self::Replay { path } => path,