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/world.npc-drop-actor-spawn.md

@@ -85,3 +85,7 @@
 ## Status
 
 `PARTIAL`。当前端已经具备 40250 实体/掉落协议的主要 native parser、VID 数据层、NPC/怪物场景层、地面物体和 shop sign 状态入口；可见性基础回归和每帧更新边界已修复，但掉落模型/特效/音效、建筑碰撞、完整 actor/ground 生命周期和 live 拾取/商店回包证据仍不足。
+
+### 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：`metin_spawn_event_system.gd`, `test_metin_spawn_event_parity.gd`。