chore: delete 106 unreachable *_system.gd and their parity tests

These files were loaded only by their own test_*_parity.gd and never by
the runtime (transitively checked, incl. client_phase_integration_system).
Removes 76 manifest implementation/evidence references across 18
contracts; contract docs get a note that conclusions based on them are
void. Live systems kept: chest, consumable, item_attr, metin_socket,
whisper_chat.

Full headless suite: no new failures (7 failing + 5 headless-hanging
render tests are identical before the deletion).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

audit/contracts/social.exchange-safebox.md

@@ -97,3 +97,7 @@
 ## Status
 
 `PARTIAL`。当前端已经具备 exchange/safebox/mall 的 native 协议骨架、回包存储和主要 UI，交易物品属性与仓库商城的基本网络状态能够展示；但 native exchange 请求缺 40250 动作门控，交易/仓库 UI 存在 optimistic 状态和提前关闭，本地 helper 直接承担交易结算、密码、库存和金条 authority，且真实错误、乱序、断线和回包回滚证据不足。本轮完成审计并记录，未修改实现。
+
+### 2026-09-22 死代码删除
+
+以下文件只被自身测试引用、运行时从不加载，已删除；本文中以它们为实现或证据的结论作废，相关行为需按 40250 源码单元重新移植：`player_trade_exchange_system.gd`, `safebox_storage_keeper_system.gd`, `test_mall_affect_parity.gd`, `test_player_trade_exchange_parity.gd`, `test_safebox_storage_keeper_parity.gd`, `test_security_lock_parity.gd`。