feat(client): protocol v6 bridge — stance, inventory, input mapping

Upgrade client protocol bridge from v5 to v6 to match server.
Adds player_stance (4 variants) and player_inventory decode to
ObserverSnapshot. Adds TOGGLE_STANCE_UP/DOWN to InputMapper.
Includes 25 gdUnit4 tests for v6 decode + server serialization
test gap fix.

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

client/tests/test_protocol.gd

@@ -280,7 +280,7 @@ func test_decode_snapshot_v2_full() -> void:
 
 	assert_that(snapshot).is_not_null()
 	assert_that(snapshot.tick).is_equal(500)
-	assert_that(snapshot.version).is_equal(4)
+	assert_that(snapshot.version).is_equal(Protocol.PROTOCOL_VERSION)
 
 	# game_time
 	assert_that(snapshot.game_time).is_not_null()
@@ -307,7 +307,7 @@ func test_existing_fixtures_have_v2_fields() -> void:
 		var bytes = _load_fixture(fixture_name)
 		var snapshot = Protocol.decode_snapshot(bytes)
 		assert_that(snapshot).is_not_null()
-		assert_that(snapshot.version).is_equal(4)
+		assert_that(snapshot.version).is_equal(Protocol.PROTOCOL_VERSION)
 		assert_that(snapshot.player_facing).is_equal("North")
 		assert_that(snapshot.game_time).is_not_null()