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.

src/event.rs

@@ -41,6 +41,20 @@ pub enum AppEvent {
     DslDropSkipped {
         command: Command,
     },
+    /// A SQL `DROP INDEX IF EXISTS` matched no index — a no-op
+    /// (ADR-0035 §4d). Renders an index-specific "doesn't exist —
+    /// skipped" note; no structure to show.
+    DslDropIndexSkipped {
+        command: Command,
+    },
+    /// A SQL `CREATE INDEX IF NOT EXISTS` matched an existing index name
+    /// — a no-op (ADR-0035 §4d). `name` is the resolved index name (the
+    /// auto-name is not on the command). Renders "already exists —
+    /// skipped"; no structure to show.
+    DslCreateIndexSkipped {
+        command: Command,
+        name: String,
+    },
     /// A `show data` query succeeded.
     DslDataSucceeded { command: Command, data: DataResult },
     /// An `explain …` command succeeded (ADR-0028). `plan`