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.mount-pet-polymorph.md

@@ -81,3 +81,7 @@
 本轮仍为 `PARTIAL`；完成坐骑、宠物和变身方向的合同审计并登记，未修改实现。
 
 `PARTIAL`。当前端已具备 `mount_vnum` 的网络存储、mount_changed 信号、部分动作模式/旋转速度/攻击门控，并有本地马匹、宠物和变身 helper；但没有发现实际坐骑模型挂接链，变身状态尚未进入 native entity，宠物与马战/成长/耐力/伤害脚本承担了 40250 客户端没有承担的本地 authority。基础测试全部通过仍不足以证明与 40250 Windows 客户端实现统一。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：`classic_pet_system.gd`, `horse_combat_system.gd`, `horse_level_growth_system.gd`, `horse_system.gd`, `mounted_combat_skill_system.gd`, `pet_companion_system.gd`, `polymorph_marble_system.gd`, `polymorph_system.gd`, `test_battle_arena_duel_parity.gd`, `test_classic_pet_parity.gd`, `test_horse_combat_parity.gd`, `test_horse_level_growth_parity.gd`, `test_mount_horse_parity.gd`, `test_mounted_combat_skill_parity.gd`, `test_pet_companion_parity.gd`, `test_polymorph_marble_parity.gd`, `test_polymorph_parity.gd`。