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/game_state.gd

@@ -25,6 +25,10 @@ var nearby_interactions: Array = []  # [{entity_id, entity_type, distance, verbs
 # v5 fields (#414)
 var current_monologue: Variant = null  # {id, text, duration_seconds} or null
 
+# v6 fields (#449, D-053, D-065)
+var player_stance: String = "Walk"  # Sprint/Walk/Careful/Crouch
+var player_inventory: Array = []  # [{item_id, name, slot}]
+
 func apply_snapshot(snapshot: Dictionary) -> void:
 	current_snapshot = snapshot
 
@@ -82,6 +86,16 @@ func apply_snapshot(snapshot: Dictionary) -> void:
 	else:
 		current_monologue = null
 
+	# v6: player_stance (#449, D-053)
+	if snapshot.has("player_stance") and snapshot.player_stance is String:
+		player_stance = snapshot.player_stance
+
+	# v6: player_inventory (#449, D-065)
+	if snapshot.has("player_inventory") and snapshot.player_inventory is Array:
+		player_inventory = snapshot.player_inventory
+	else:
+		player_inventory = []
+
 	# v2: visible_tiles with visibility sectors
 	# Derives visible_positions when not explicitly provided (real server mode)
 	if snapshot.has("visible_tiles") and snapshot.visible_tiles is Array and snapshot.visible_tiles.size() > 0: