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/audio.bgm-ambience-3d.md

@@ -102,3 +102,7 @@
 
 - 先审计 `lifecycle.platform_release`，核对 Windows 窗口、导出、自包含资源、崩溃处理和发布门禁。
 - 回到实现阶段时，优先建立 BGM 多实例状态机和真实 3D 播放器清理测试，再处理固定实例池/Provider 的适配边界。
+
+### 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：`test_bgm_jukebox_parity.gd`。