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/completion.rs

@@ -2073,9 +2073,12 @@ mod tests {
 
     #[test]
     fn new_name_slot_offers_no_candidates_even_with_populated_cache() {
-        // `create table ` — the table-name slot is NewName.
-        // Even if the cache has table/column entries, no
-        // candidates are offered (the user invents the name).
+        // `create table ` — the table-name slot is NewName, so even
+        // with a populated cache no *schema* candidates are offered
+        // (the user invents the name). In advanced mode the sole
+        // candidate here is the optional `if` keyword (the
+        // `IF NOT EXISTS` prefix, ADR-0035 §4) — never a cached
+        // table/column name.
         let cache = SchemaCache {
             tables: vec!["Existing".to_string()],
             columns: vec!["AlsoExisting".to_string()],
@@ -2083,7 +2086,11 @@ mod tests {
             ..SchemaCache::default()
         };
         let cs = cands_with("create table ", 13, &cache);
-        assert!(cs.is_empty(), "got {cs:?}");
+        assert!(
+            !cs.iter().any(|c| c == "Existing" || c == "AlsoExisting"),
+            "NewName slot must not surface schema candidates; got {cs:?}"
+        );
+        assert_eq!(cs, vec!["if".to_string()], "only the advanced IF NOT EXISTS keyword");
     }
 
     fn keyword_cand(text: &str) -> Candidate {