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>

server/tests/serialization.rs

@@ -90,6 +90,9 @@ fn all_player_action_variants_roundtrip() {
         PlayerAction::UsePerceptionMode("thermal".to_string()),
         PlayerAction::Pause,
         PlayerAction::Unpause,
+        PlayerAction::SetTickRate(TickRate::Half),
+        PlayerAction::ToggleStanceUp,
+        PlayerAction::ToggleStanceDown,
     ];
 
     for action in actions {