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/friendly/strings/en-US.yaml

@@ -265,6 +265,11 @@ help:
           [, primary key (<col>, ...)])    — create a table (advanced SQL)
     sql_drop_table: |-
         drop table [if exists] <T>                       — remove a table (advanced SQL)
+    sql_create_index: |-
+        create [unique] index [if not exists] [<name>] on <T> (<col>, ...)
+                                                         — create an index (advanced SQL)
+    sql_drop_index: |-
+        drop index [if exists] <name>                    — remove an index (advanced SQL)
     drop: |-
         drop table <T>                                  — remove a table
         drop column [from] [table] <T>: <col> [--cascade]   — remove a column
@@ -382,6 +387,12 @@ ddl:
   # `drop table if exists <T>` where the table is absent: a no-op that
   # succeeds with this note instead of a "doesn't exist" error.
   drop_skipped_absent: "table '{name}' doesn't exist — skipped (no changes made)"
+  # `create [unique] index if not exists <name> …` where the index name
+  # already exists: a no-op that succeeds with this note (ADR-0035 §4d).
+  create_index_skipped_exists: "index '{name}' already exists — skipped (no changes made)"
+  # `drop index if exists <name>` where the index is absent: a no-op that
+  # succeeds with this note instead of a "doesn't exist" error.
+  drop_index_skipped_absent: "index '{name}' doesn't exist — skipped (no changes made)"
 
 parse:
   # Wrapper around chumsky's structural error message. The
@@ -452,6 +463,8 @@ parse:
     create_table: "create table <Name> with pk [<col>(<type>)[, ...]]"
     sql_create_table: "create table [if not exists] <Name> (<col> <type> [not null] [unique] [primary key], ... [, primary key (<col>, ...)])"
     sql_drop_table: "drop table [if exists] <Name>"
+    sql_create_index: "create [unique] index [if not exists] [<Name>] on <Table> (<col>[, ...])"
+    sql_drop_index: "drop index [if exists] <Name>"
     drop_table: "drop table <Name>"
     drop_column: "drop column [from] [table] <Table>: <Name>"
     drop_relationship: |-