fix(protocol): drop PROTOCOL_VERSION lockstep — D-192 (#875)

Removes the version-mismatch guard from Protocol.decode_snapshot() and the
PROTOCOL_VERSION constant from the client (server side done in #874).

Core changes:
- protocol.gd: remove const PROTOCOL_VERSION, remove version mismatch guard,
  remove "version" from return dict, add gauntlet_mode/room_id decode
- sim_bridge.gd: remove handshake version check; relax handshake guard to
  require only a valid Dictionary (server no longer sends protocol_version);
  emit handshake_complete(0) for API compat
- loading_screen.gd: drop "· protocol N" suffix from version label
- test_harness.gd: replace Protocol.PROTOCOL_VERSION with literal 23

Test updates (21 files): replace "version": Protocol.PROTOCOL_VERSION with
"version": 23 in all snapshot bytes dicts; remove snapshot.version == N
assertions; remove version-rejection tests (test_rejects_version_6,
test_decode_snapshot_rejects_missing_version, test_decode_snapshot_rejects_old_version,
test_protocol_rejects_version_mismatch, test_sim_bridge_test_snapshot_uses_current_protocol_version).

Also includes: #872 bookmark_catalog carry-forward regression test, and
#873 merge-path flow tests (test_merge_path_flows_sprint37.gd).

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

client/tests/test_interaction_list.gd

@@ -305,7 +305,7 @@ func test_contradicted_entity_verbs_decode() -> void:
 	# #422: NearbyInteraction.contradicted=true should be decodeable
 	var raw := {
 		"tick": 1,
-		"version": Protocol.PROTOCOL_VERSION,
+		"version": 23,
 		"entities": [],
 		"nearby_interactions": [{
 			"entity_id": 2,
@@ -329,7 +329,7 @@ func test_object_type_verbs_decode() -> void:
 	# #421: ObjectType appears in NearbyInteraction.object_type
 	var raw := {
 		"tick": 1,
-		"version": Protocol.PROTOCOL_VERSION,
+		"version": 23,
 		"entities": [],
 		"nearby_interactions": [{
 			"entity_id": 3,