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/protocol/protocol.gd

@@ -11,7 +11,7 @@ class_name Protocol
 
 ## Protocol version — must match server PROTOCOL_VERSION in bridge/types.rs.
 ## Reject snapshots where version != this value.
-const PROTOCOL_VERSION: int = 5
+const PROTOCOL_VERSION: int = 6
 
 
 # -- Decode: bytes from server → GDScript types --------------------------------
@@ -104,6 +104,24 @@ static func decode_snapshot(bytes: PackedByteArray) -> Variant:
 			"duration_seconds": float(raw_monologue.get("duration_seconds", 5.0)),
 		}
 
+	# v6: player_stance (#449, D-053) — unit enum → bare string
+	var player_stance: String = "Walk"
+	var raw_stance: Variant = raw.get("player_stance")
+	if raw_stance is String:
+		player_stance = raw_stance
+
+	# v6: player_inventory (#449, D-065) — array of {item_id, name, slot}
+	var player_inventory: Array = []
+	var raw_inventory: Variant = raw.get("player_inventory")
+	if raw_inventory is Array:
+		for raw_item in raw_inventory:
+			if raw_item is Dictionary and raw_item.has("item_id") and raw_item.has("name"):
+				player_inventory.append({
+					"item_id": int(raw_item["item_id"]),
+					"name": str(raw_item["name"]),
+					"slot": int(raw_item.get("slot", 0)),
+				})
+
 	return {
 		"tick": tick,
 		"entities": entities,
@@ -111,6 +129,8 @@ static func decode_snapshot(bytes: PackedByteArray) -> Variant:
 		"version": version,
 		"game_time": game_time,
 		"player_facing": player_facing,
+		"player_stance": player_stance,
+		"player_inventory": player_inventory,
 		"visible_tiles": visible_tiles,
 		"nearby_interactions": nearby_interactions,
 		"current_monologue": current_monologue,