feat: ADR-0035 4c — DROP TABLE [IF EXISTS]

Add advanced-mode SQL `DROP TABLE [IF EXISTS] <name>` -> SqlDropTable,
executing through the existing do_drop_table (cascade / inbound-
relationship refusal / metadata cleanup) — full parity with the simple
`drop table`. The only new behaviour is `IF EXISTS` as a
no-op-with-note: a new DropOutcome::Skipped mirroring
CreateOutcome::Skipped (journalled, no snapshot), rendered via a new
ddl.drop_skipped_absent note + DslDropSkipped event.

- Grammar: SQL_DROP_TABLE node (entry `drop`, shape `table [if exists]
  <name> [;]`), registered Advanced. SQL-first dispatch: `drop table T`
  -> SqlDropTable in advanced; `drop column`/`relationship`/`index`/
  `constraint` fall back to the simple `drop` node (and still execute).
- Worker: Request::SqlDropTable + db.sql_drop_table; the if-exists-and-
  absent arm journals + replies Skipped without a snapshot, else
  snapshot_then(do_drop_table) -> Dropped.
- Completion: advanced `drop ` now surfaces the SQL `table` (the
  shared-entry-word behaviour from `create`); test split into simple
  (full DSL list) + advanced (SQL surface).

Known shared-entry-word completion unevenness (advanced `drop ` offers
only `table`; partial `drop rel` returns an empty list) deferred to 4i
(merge candidate sets for shared entry words) along with a flagged user
request to visually distinguish simple- vs advanced-mode completions in
the hint UI — tracked in ADR §13 4i (d)/(e), the 4c plan, and the
completion test. The DSL drops still parse + execute via fallback.

10 new tests (parse/builder + Tier-3: drop existing + one-undo-step +
restore, IF EXISTS skip + journal, plain-absent error, inbound refusal).
Docs: ADR-0035 Status/§13, README, requirements.md Q1.

Tests: 1805 passing, 0 failing, 1 ignored. Clippy clean.

docs/requirements.md

@@ -223,8 +223,9 @@ handoff-14 cleanup; 449 after B2/C2.)
   table, since the engine reports no CHECK constraints), then foreign
   keys (4b — inline `REFERENCES` + table-level `FOREIGN KEY` → ADR-0013
   named relationships in the create transaction; self-references and
-  bare `REFERENCES <parent>` supported)). Remaining DDL — `DROP TABLE`
-  (4c), indexes (4d), `ALTER TABLE` (4e–4h) — is phased per
+  bare `REFERENCES <parent>` supported), then `DROP TABLE [IF EXISTS]`
+  (4c — reuses `do_drop_table`; `IF EXISTS` is a no-op-with-note)).
+  Remaining DDL — indexes (4d), `ALTER TABLE` (4e–4h) — is phased per
   ADR-0035 §13.)*
 - [ ] **Q2** Non-standard syntax rejected with a clear message
   pointing at the supported subset.