feat(client): permanent dialogue panel + protocol v13 + build check

- dialogue_box.gd: panel is always visible as permanent insert UI
  element per D-061 — content fades but frame stays on screen
- protocol.gd: bump PROTOCOL_VERSION to 13
- Makefile: add check-protocol target that verifies server/client
  protocol versions match, runs automatically before build

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

client/scripts/protocol/protocol.gd

@@ -11,7 +11,7 @@ class_name Protocol
 
 ## Protocol version — must match server PROTOCOL_VERSION in bridge/types.rs.
 ## Reject snapshots where version != this value.
-const PROTOCOL_VERSION: int = 12
+const PROTOCOL_VERSION: int = 13
 
 
 # -- Decode: bytes from server → GDScript types --------------------------------