docs: ADR-0030 — advanced mode standard-SQL surface

Decides the architecture for SQL in advanced mode (Q1/Q2/Q4):
SQL is authored as grammar within the unified grammar tree
(ADR-0024) and parsed by the existing walker — not a separate
batch parser — so SQL gets the same completion, highlighting,
hints, and parse-error reporting as the DSL. Mode gates the
SQL forms. DDL routes through the typed Command executor
(metadata and the playground type vocabulary preserved); DML
and SELECT execute as validated SQL. Engine-neutral posture;
DSL→SQL teaching echo; phased plan.

Supersedes ADR-0001's sqlparser-rs reservation. Ticks Q4;
updates the ADR index and the Q1/Q2 notes. handoff-24 orients
the implementation session at Phase 1.

docs/requirements.md

@@ -196,15 +196,31 @@ handoff-14 cleanup; 449 after B2/C2.)
 
 - [ ] **Q1** SQL parsed via `sqlparser-rs`; supported subset is
   defined (specifics deferred to a future ADR).
-  *(Progress: DSL is parsed via `chumsky` (ADR-0009); SQL
-  handling in advanced mode is still a placeholder echo.)*
+  *(Progress: SQL handling in advanced mode is still a
+  placeholder echo. The architecture is now decided — ADR-0030:
+  SQL is authored as grammar within the unified grammar tree
+  (ADR-0024) and parsed by the existing walker, **not** a
+  separate batch parser — so SQL gets the same completion /
+  highlighting / hints as the DSL. ADR-0001's `sqlparser-rs`
+  reservation is superseded. Implementation is phased and
+  pending.)*
 - [ ] **Q2** Non-standard syntax rejected with a clear message
   pointing at the supported subset.
+  *(Design done — ADR-0030 §8: out-of-subset statements are
+  refused with an engine-neutral message naming the construct.
+  Implementation pending.)*
 - [x] **Q3** User-facing simplified types map transparently to
   SQLite STRICT types in generated DDL. *(All ten types implemented
   and tested.)*
-- [~] **Q4** Supported SQL subset specification — design and ADR
-  pending. Q1 cannot be marked satisfied without it.
+- [x] **Q4** Supported SQL subset specification — **ADR-0030**.
+  Advanced mode is a standard-SQL surface, engine-neutral; the
+  supported surface — `SELECT` (full query surface),
+  `INSERT` / `UPDATE` / `DELETE`, `CREATE` / `DROP` / `ALTER
+  TABLE`, `CREATE` / `DROP INDEX` — is authored as grammar in
+  the unified tree. DDL routes through the typed `Command`
+  executor (metadata + the playground type vocabulary
+  preserved); DML and `SELECT` execute as validated SQL. Q1's
+  implementation is now unblocked.
 
 ## Database backend (per ADR-0002)