feat: ADR-0035 4d — CREATE [UNIQUE] INDEX / DROP INDEX

Advanced-mode SQL CREATE [UNIQUE] INDEX [IF NOT EXISTS] [<name>] ON
<T> (cols) -> SqlCreateIndex and DROP INDEX [IF EXISTS] <name> ->
SqlDropIndex, both reusing the ADR-0025 executors (do_add_index /
do_drop_index), like 4c reused do_drop_table.

- CREATE UNIQUE INDEX admitted in advanced mode (ADR-0025 Amendment 1):
  ADR-0025 deferred UNIQUE indexes for the simple-mode DSL, but advanced
  mode trusts the user like SQL does. Adds an additive IndexSchema.unique
  flag (project.yaml, serde-default, version stays 1); rebuild re-emits
  CREATE UNIQUE INDEX; the redundant-set guard keys on (columns, unique).
  Simple-mode `add unique index` stays deferred.
- IF [NOT] EXISTS on both forms reuses the 4c no-op-with-note skip
  (journalled, not snapshotted) via CreateIndexOutcome / DropIndexOutcome.
- Unnamed CREATE INDEX auto-named (ADR-0025 convention); the [UNIQUE]
  prefix is a concrete-keyword Choice and the optional name an on-led-first
  selector (the drop-index selector precedent) — trap-safe.
- create/drop each gain a second advanced node; the existing all-candidates
  dispatch handles it (locked by parse tests).
- Unique indexes marked [unique] in the structure view and items panel.
- do_add_index refuses internal __rdbms_* tables as "no such table",
  closing a latent exposure on both the simple `add index` and the new
  SQL CREATE INDEX surfaces (ADR-0025 Amendment 1).

Docs: ADR-0035 status + §13 4d + 4i; ADR-0025 Amendment 1; ADR README;
requirements.md Q1/C3. Plan: docs/plans/20260525-adr-0035-sql-ddl-4d.md.

Tests: 1834 passing / 0 failing / 0 skipped / 1 ignored; clippy clean.

docs/requirements.md

@@ -168,7 +168,10 @@ handoff-14 cleanup; 449 after B2/C2.)
   inbound view in the structure renderer; type compatibility
   validated at declaration via `Type::fk_target_type()`.
   Indexes done (ADR-0025) — `add index` / `drop index`,
-  rebuild-preserving, persisted in `project.yaml`.
+  rebuild-preserving, persisted in `project.yaml`; UNIQUE indexes
+  added on the advanced-mode SQL surface (`CREATE UNIQUE INDEX`,
+  ADR-0035 §4d / ADR-0025 Amendment 1; simple-mode `add unique index`
+  deferred).
   `NOT NULL` / `UNIQUE` / `CHECK` / `DEFAULT` done (ADR-0029) —
   a constraint suffix on `create table` / `add column`, plus
   `add constraint` / `drop constraint` on existing columns;
@@ -224,9 +227,11 @@ handoff-14 cleanup; 449 after B2/C2.)
   keys (4b — inline `REFERENCES` + table-level `FOREIGN KEY` → ADR-0013
   named relationships in the create transaction; self-references and
   bare `REFERENCES <parent>` supported), then `DROP TABLE [IF EXISTS]`
-  (4c — reuses `do_drop_table`; `IF EXISTS` is a no-op-with-note)).
-  Remaining DDL — indexes (4d), `ALTER TABLE` (4e–4h) — is phased per
-  ADR-0035 §13.)*
+  (4c — reuses `do_drop_table`; `IF EXISTS` is a no-op-with-note), then
+  `CREATE [UNIQUE] INDEX` / `DROP INDEX [IF EXISTS]` (4d — reuse
+  `do_add_index`/`do_drop_index`; `CREATE UNIQUE INDEX` admitted in
+  advanced mode via the `IndexSchema.unique` flag, ADR-0025 Amendment 1)).
+  Remaining DDL — `ALTER TABLE` (4e–4h) — is phased per ADR-0035 §13.)*
 - [ ] **Q2** Non-standard syntax rejected with a clear message
   pointing at the supported subset.
   *(Design done — ADR-0030 §8: out-of-subset statements are