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/friendly/keys.rs

@@ -172,8 +172,10 @@ pub const KEYS_AND_PLACEHOLDERS: &[(&str, &[&str])] = &[
     ("help.app.redo", &[]),
     ("help.ddl.create", &[]),
     ("help.ddl.sql_create_table", &[]),
-    // Advanced-mode SQL CREATE TABLE no-op note (ADR-0035 §4).
+    ("help.ddl.sql_drop_table", &[]),
+    // Advanced-mode SQL CREATE TABLE / DROP TABLE no-op notes (ADR-0035 §4).
     ("ddl.create_skipped_exists", &["name"]),
+    ("ddl.drop_skipped_absent", &["name"]),
     ("help.ddl.drop", &[]),
     ("help.ddl.add", &[]),
     ("help.ddl.rename", &[]),
@@ -245,6 +247,7 @@ pub const KEYS_AND_PLACEHOLDERS: &[(&str, &[&str])] = &[
     ("parse.usage.change_column", &[]),
     ("parse.usage.create_table", &[]),
     ("parse.usage.sql_create_table", &[]),
+    ("parse.usage.sql_drop_table", &[]),
     ("parse.usage.delete", &[]),
     ("parse.usage.drop_column", &[]),
     ("parse.usage.drop_constraint", &[]),