feat: ADR-0035 4a — SQL CREATE TABLE command, worker, and exit gate

Command + builder + worker for advanced-mode SQL CREATE TABLE
(sub-phase 4a), executed structurally through do_create_table:

- Command::SqlCreateTable + build_sql_create_table (ddl.rs): aliases via
  from_sql_name (incl. double precision), column- and table-level
  PRIMARY KEY, redundant-flag de-dup off a sole PK, IF NOT EXISTS.
  Advanced REGISTRY entry on the shared `create` word (SQL-first, DSL
  fallback); no-PK tables allowed (user-confirmed).
- Worker (db.rs): Request::SqlCreateTable + CreateOutcome + snapshot_then
  (one undo step); IF NOT EXISTS no-op (no snapshot, but journalled, like
  read-only commands). do_create_table inline-PK rule aligned with the
  rebuild generator schema_to_ddl — no round-trip DDL drift; serial
  autoincrement is independent of inline-PK (verified by round-trip
  tests).
- Runtime/App: dispatch + CommandOutcome::SchemaSkipped +
  AppEvent::DslCreateSkipped (structure + "already exists — skipped"
  note). Friendly catalog keys added (engine-neutral).

DEFAULT/CHECK/table-level UNIQUE are absent from the 4a grammar (parse
error with usage skeleton; friendly message + support land in the 4a.2
constraint slice) — user-confirmed.

Tests: type resolver, grammar shape, builder (incl. the PK
detection bug they caught), and tests/sql_create_table.rs (worker
round-trip, serial autoincrement first/non-first across rebuild, IF NOT
EXISTS no-op + journalling, no-PK table, one undo step) + a replay-as-
write test. 1739 pass / 0 fail / 1 ignored; clippy clean.

Exit gate: ADR-0035 Proposed -> Accepted (validated end-to-end by 4a);
README + requirements.md Q1 updated.

src/event.rs

@@ -28,6 +28,13 @@ pub enum AppEvent {
         command: Command,
         description: Option<TableDescription>,
     },
+    /// A SQL `CREATE TABLE IF NOT EXISTS` matched an existing table —
+    /// a no-op (ADR-0035 §4). Renders the existing structure plus an
+    /// "already exists — skipped" note.
+    DslCreateSkipped {
+        command: Command,
+        description: TableDescription,
+    },
     /// A `show data` query succeeded.
     DslDataSucceeded { command: Command, data: DataResult },
     /// An `explain …` command succeeded (ADR-0028). `plan`