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/scripts/autoloads/sim_bridge.gd

@@ -242,6 +242,8 @@ static func _action_enum_to_wire(action: int) -> String:
 		InputMapper.Action.INTERACT: return "Interact"
 		InputMapper.Action.USE_PERCEPTION_MODE: return "UsePerceptionMode"
 		InputMapper.Action.PAUSE: return "Pause"
+		InputMapper.Action.TOGGLE_STANCE_UP: return "ToggleStanceUp"
+		InputMapper.Action.TOGGLE_STANCE_DOWN: return "ToggleStanceDown"
 		InputMapper.Action.OPEN_MENU:
 			# Client-only action, not part of wire protocol
 			push_warning("SimBridge: OPEN_MENU is client-only, not sent to server")
@@ -328,6 +330,8 @@ func _test_snapshot() -> Dictionary:
 			"tick_rate": "Full",
 		},
 		"player_facing": _test_facing,
+		"player_stance": "Walk",
+		"player_inventory": [],
 		"entities": entities,
 		"tiles": _test_tiles(),
 		"visible_tiles": _test_visible_tiles(),