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/rendering/world_renderer.gd

@@ -15,6 +15,6 @@ func update_from_state() -> void:
 	if entity_renderer and entity_renderer.has_method("update_entities"):
 		entity_renderer.update_entities(GameState.visible_entities)
 
-	# Update fog overlay
+	# Update fog overlay (fog data will come in D-020 expansion)
 	if fog_renderer and fog_renderer.has_method("update_fog"):
-		fog_renderer.update_fog(GameState.fog_state, GameState.player_position)
+		fog_renderer.update_fog({}, GameState.player_position)