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_bug_report_ring_buffer.gd

@@ -14,7 +14,7 @@ extends GdUnitTestSuite
 # Expected ring buffer capacity per spec.
 const EXPECTED_CAPACITY := 60
 
-var BugReportDialogScript = load("res://ui/bug_report_dialog.gd")
+const BugReportDialogScene = preload("res://ui/bug_report_dialog.tscn")
 
 
 func after_each() -> void:
@@ -35,8 +35,10 @@ func after_each() -> void:
 # -- Helpers -------------------------------------------------------------------
 
 func _make_dialog() -> Control:
-	var dialog = Control.new()
-	dialog.set_script(BugReportDialogScript)
+	# Instantiate via .tscn — preserves the MetaScreen runtime stack.
+	# (Sprint 36 migrated bug_report_dialog.gd to extends MetaScreen; bare
+	# Control.new() + set_script() no longer satisfies the base contract.)
+	var dialog: Control = BugReportDialogScene.instantiate()
 	auto_free(dialog)
 	add_child(dialog)
 	return dialog
@@ -52,7 +54,7 @@ func _make_input(tick: int, action: String = "MoveNorth") -> Dictionary:
 func _make_snapshot_json(tick: int) -> String:
 	return JSON.stringify({
 		"tick": tick,
-		"version": Protocol.PROTOCOL_VERSION,
+		"version": 23,
 		"entities": [],
 	})