feat(client): protocol v23 — bookmark_catalog decode + bookmark actions (Workstream 3)

Adds client-side wire support for the bookmark catalog (#614) and the
two associated player actions. PROTOCOL_VERSION bumps from 21 to 23:
- v22 (server): RequestBookmarkCatalog + ConfirmBookmark player actions
- v23 (server): bookmark_catalog field on ObserverSnapshot

Decode:
- protocol.gd decode_snapshot extracts optional bookmark_catalog.
  Defensive parse of BookmarkWire fields (id, title, subtitle, flavor,
  default_location, allowed_locations, allowed_locations_cultures,
  career, starting_capital_tractus). Missing or malformed → null.
- snapshot_handler.gd caches the catalog into GameState.bookmark_catalog
  on each snapshot (server pushes on tick 0; re-fetchable via
  RequestBookmarkCatalog).
- GameState gains bookmark_catalog: Array = [] (untyped per autoload
  parse-order discipline; default empty so callers can iterate without
  null checks).

Encode:
- encode_request_bookmark_catalog() — unit variant, sent to trigger a
  re-push if the cached catalog is missing.
- encode_confirm_bookmark(bookmark_id, starting_location_id) — struct
  variant matching server rmp_serde shape. Called from character
  creation on Start (lands in Workstream 6).

Tests:
- 5 new cases in test_protocol.gd: hand-built bookmark_catalog decode
  (all 9 fields asserted), fixture-based decode round-trip, missing-
  field null behavior, RequestBookmarkCatalog encode roundtrip,
  ConfirmBookmark encode roundtrip.
- All 12 existing snapshot fixtures regenerated from server via
  `cargo test --test gen_fixtures -- --ignored`. The new
  snapshot_with_bookmark_catalog.msgpack fixture was generated by the
  same pass.

Verification:
- gdlint clean
- godot --headless --path client --quit — no SCRIPT ERROR
- test_protocol 62/62, test_client_p3 24/24, test_implant_nav_stack
  52/52, test_implant_registry 42/42, test_implant_app_lifecycle 36/36

Workstream 4 (Option A sequencing via loading_screen + SimBridge
connect) lands next.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

client/scripts/autoloads/game_state.gd

@@ -122,6 +122,13 @@ var settings_response: Variant = null
 # Null when no economy data in the current snapshot.
 var economy_snapshot: Variant = null
 
+# v23 fields (#614): Bookmark catalog from server.
+# One-shot response to RequestBookmarkCatalog. Array of bookmark Dictionaries:
+# [{id, title, subtitle, flavor, default_location, allowed_locations,
+#   allowed_locations_cultures, career, starting_capital_tractus}]
+# Empty array when no catalog has been received yet.
+var bookmark_catalog: Array = []
+
 # v7 fields (#431, D-059/D-060)
 var pending_recognitions: Array = []  # [{entity_id, x, y, z, remaining_ticks, total_delay_ticks}]