fix(client): address PR #4 review feedback

Hoshe critical fixes:
- _action_enum_to_wire uses InputMapper.Action constants instead of
  fragile integer literals; OPEN_MENU explicitly handled as client-only
- Remove int() coercion on tick/entity_id — use direct assignment since
  GDScript int is signed 64-bit (safe for realistic tick values)
- Check encode result before buffering in send_input() — reject empty
  bytes instead of corrupting the outbound stream
- Test snapshot now uses Protocol format {tick, entities} instead of
  legacy schema; GameState updated to derive player position from
  entity data; main.gd and world_renderer.gd updated accordingly

Hoshe warnings:
- 5 negative tests added (truncated bytes, wrong type, missing fields,
  empty bytes, encode validation) — 20/20 tests pass
- receive_bytes signal is emitted at consume time in poll_snapshot by
  design (documented in code)

Tyre suggestions:
- Remove duplicated root-level fixtures — single source of truth in
  client/tests/fixtures/msgpack/
- gen_fixtures.rs writes directly to client/ directory
- Add `make fixtures` target for regeneration

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

client/scripts/protocol/protocol.gd

@@ -31,8 +31,11 @@ static func decode_snapshot(bytes: PackedByteArray) -> Variant:
 		if entity != null:
 			entities.append(entity)
 
+	# GDScript int is signed 64-bit. Rust tick is u64 but will not exceed 2^63
+	# in any realistic scenario (would require ~29 billion years at 10 ticks/min).
+	var tick: int = raw["tick"]
 	return {
-		"tick": int(raw["tick"]),
+		"tick": tick,
 		"entities": entities,
 	}
 
@@ -44,8 +47,9 @@ static func _decode_entity(raw: Dictionary) -> Variant:
 		push_warning("Protocol: entity missing required fields: %s" % str(raw.keys()))
 		return null
 
+	var entity_id: int = raw["entity_id"]
 	return {
-		"entity_id": int(raw["entity_id"]),
+		"entity_id": entity_id,
 		"x": float(raw["x"]),
 		"y": float(raw["y"]),
 		"z": int(raw["z"]),
@@ -108,7 +112,8 @@ static func decode_player_input(bytes: PackedByteArray) -> Variant:
 		push_error("Protocol: player_input missing required fields")
 		return null
 
+	var tick: int = raw["tick"]
 	return {
-		"tick": int(raw["tick"]),
+		"tick": tick,
 		"action": _decode_enum_variant(raw["action"]),
 	}

client/scripts/protocol/protocol.gd.uid

@@ -0,0 +1 @@
+uid://fkmd537xvwxe