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.

src/completion.rs

@@ -1458,12 +1458,14 @@ mod tests {
     }
 
     #[test]
-    fn drop_offers_all_five_subcommands() {
-        // `drop` branches: column / relationship / table / index
-        // (ADR-0025) / constraint (ADR-0029 §2.2). Candidates
-        // follow grammar declaration order, so `constraint` —
-        // added last — appears last.
-        let cs = cands("drop ", 5);
+    fn drop_offers_all_five_subcommands_in_simple_mode() {
+        // The DSL `drop` branches: column / relationship / table / index
+        // (ADR-0025) / constraint (ADR-0029 §2.2). Candidates follow
+        // grammar declaration order, so `constraint` — added last —
+        // appears last. Simple mode, because `drop` is a shared entry
+        // word: advanced mode surfaces the SQL `DROP TABLE` completion
+        // instead (ADR-0033 Amendment 3 / ADR-0035 §4c — see below).
+        let cs = cands_simple("drop ", 5);
         assert_eq!(
             cs,
             vec![
@@ -1476,6 +1478,23 @@ mod tests {
         );
     }
 
+    #[test]
+    fn drop_in_advanced_mode_surfaces_the_sql_drop_table_completion() {
+        // ADR-0035 §4c: `drop` gained an advanced SQL node
+        // (`DROP TABLE [IF EXISTS]`). As with the `create`/`insert`/
+        // `update`/`delete` shared entry words (ADR-0033 Amendment 3),
+        // advanced mode surfaces the SQL grammar's completion — here
+        // just `table` — rather than the DSL subcommands. The DSL drops
+        // (`drop column` etc.) still parse via fallback; only the
+        // completion hint differs (and a partial DSL keyword like
+        // `drop rel` returns an empty list — a mid-word dead end).
+        // ADR-0035 §13 4i (d)/(e) tracks merging the candidate sets for
+        // shared entry words, and the user's request to visually
+        // distinguish simple- vs advanced-mode completions in the hint
+        // UI (likely by colour); this expectation grows when 4i lands.
+        assert_eq!(cands("drop ", 5), vec!["table".to_string()]);
+    }
+
     #[test]
     fn complete_command_offers_no_candidates() {
         // `create table T with pk` is a complete command —
@@ -2444,3 +2463,5 @@ mod tests {
         assert!(candidates_at_cursor_with("create ", 7, &cache, empty_ranker).is_none());
     }
 }
+
+