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>