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/output_render.rs

@@ -137,12 +137,15 @@ pub fn render_structure(desc: &TableDescription) -> Vec<String> {
     }
 
     // Indexes section (ADR-0025), shown only when the table
-    // carries at least one user-created index.
+    // carries at least one user-created index. A UNIQUE index is
+    // marked `[unique]` so a learner can tell a uniqueness-enforcing
+    // index from a performance-only one (ADR-0035 §4d).
     if !desc.indexes.is_empty() {
         out.push("Indexes:".to_string());
         for index in &desc.indexes {
+            let unique = if index.unique { " [unique]" } else { "" };
             out.push(format!(
-                "  {} ({})",
+                "  {} ({}){unique}",
                 index.name,
                 index.columns.join(", "),
             ));
@@ -797,6 +800,30 @@ mod tests {
         let out = render_structure(&desc).join("\n");
         assert!(out.contains("Indexes:"), "got:\n{out}");
         assert!(out.contains("idx_email (Email)"), "got:\n{out}");
+        // A plain index carries no uniqueness marker.
+        assert!(!out.contains("[unique]"), "plain index unmarked; got:\n{out}");
+    }
+
+    #[test]
+    fn render_structure_marks_a_unique_index() {
+        // ADR-0035 §4d: a UNIQUE index is marked `[unique]` so a learner
+        // can tell it from a performance-only index.
+        let desc = TableDescription {
+            name: "Customers".to_string(),
+            columns: vec![
+                col("id", Type::Serial, true, false),
+                col("Email", Type::Text, false, false),
+            ],
+            outbound_relationships: Vec::new(),
+            inbound_relationships: Vec::new(),
+            indexes: vec![IndexInfo {
+                name: "uidx_email".to_string(),
+                columns: vec!["Email".to_string()],
+                unique: true,
+            }],
+        };
+        let out = render_structure(&desc).join("\n");
+        assert!(out.contains("uidx_email (Email) [unique]"), "got:\n{out}");
     }
 
     #[test]