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/app.rs

@@ -459,6 +459,16 @@ impl App {
                 self.current_table = Some(description);
                 Vec::new()
             }
+            AppEvent::DslDropSkipped { command } => {
+                // No-op (DROP TABLE IF EXISTS on an absent table,
+                // ADR-0035 §4, 4c): just the skip note — no structure,
+                // no misleading "[ok] drop table" line.
+                self.note_system(crate::t!(
+                    "ddl.drop_skipped_absent",
+                    name = command.target_table()
+                ));
+                Vec::new()
+            }
             AppEvent::DslDataSucceeded { command, data } => {
                 self.handle_dsl_query_success(&command, &data);
                 Vec::new()
@@ -1554,6 +1564,7 @@ impl App {
                 (Operation::CreateTable, Some(name.as_str()), None)
             }
             C::DropTable { name } => (Operation::DropTable, Some(name.as_str()), None),
+            C::SqlDropTable { name, .. } => (Operation::DropTable, Some(name.as_str()), None),
             C::AddColumn { table, column, .. } => (
                 Operation::AddColumn,
                 Some(table.as_str()),