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/app.rs

@@ -1574,10 +1574,30 @@ impl App {
         command: &Command,
         facts: crate::friendly::FailureContext,
     ) -> crate::friendly::TranslateContext {
-        use crate::dsl::{Command as C, IndexSelector, RelationshipSelector};
+        use crate::dsl::{AlterTableAction, Command as C, IndexSelector, RelationshipSelector};
         use crate::friendly::{Operation, TranslateContext};
         let (operation, fallback_table, fallback_column) = match command {
             C::CreateTable { name, .. } => (Operation::CreateTable, Some(name.as_str()), None),
+            // SQL `ALTER TABLE` routes engine/validation errors through
+            // the operation matching its action, with the parsed table
+            // (and column, where the action names one) — ADR-0035 §4e.
+            C::SqlAlterTable { table, action } => match action {
+                AlterTableAction::AddColumn(spec) => (
+                    Operation::AddColumn,
+                    Some(table.as_str()),
+                    Some(spec.name.as_str()),
+                ),
+                AlterTableAction::DropColumn { column } => (
+                    Operation::DropColumn,
+                    Some(table.as_str()),
+                    Some(column.as_str()),
+                ),
+                AlterTableAction::RenameColumn { old, .. } => (
+                    Operation::RenameColumn,
+                    Some(table.as_str()),
+                    Some(old.as_str()),
+                ),
+            },
             C::SqlCreateTable { name, .. } => {
                 (Operation::CreateTable, Some(name.as_str()), None)
             }