feat(client): 删除 rc.7 宿主的 __DSH_MODULES__ 回退路径

peer 基线已是 ^0.1.2-rc.1（全部 dsh-* peer），最后提供
window.__DSH_MODULES__ 页面全局的宿主是 0.1.0-rc.7，远低于下限，
装不上本插件——回退在受支持宿主上不可达。受支持宿主上 apply() 在
任何懒加载发生前调 setChunkModuleSystem(ctx.modules)，moduleSystem()
前两跳（注入变量 + 插件全局 __dshSidebarModuleSystem__）必中。

回退的唯一活消费者是 vitest fixture：installModuleSystem() 原先经
rc.7 全局投递 fake 模块系统，现改为与生产同路径的 setChunkModuleSystem
注入；无任何专测回退分支的用例，全部断言保持不变。

- chunk-loader.ts: 删 moduleSystem() 第三跳，同步头注 /
  ChunkModuleSystem 接口注中的回退描述
- index.tsx: 删「falls back to the rc.7 global」注释
- README v0.14.0 changelog 与 docs/plans 设计文档中的提及属历史
  存档（描述 PR #232 当时的迁移），保留不改

验证：lint / typecheck / build / vitest 119 文件 1243 用例全绿；
真机挂载 lane（钉版 @deepseek-ai/dsh@0.1.2-rc.1 CLI）mount.e2e
全内置 tab sweep（含终端 / editor 懒加载 chunk）通过；lib/*.js
产物中无 __DSH_MODULES__ 残留。

src/client/chunk-loader.ts

@@ -23,8 +23,9 @@
  *    arbitrary file names, so the plugin's own host route serves the chunks),
  * 2. read the factory from the global registry,
  * 3. call it with a require that resolves the platform externals through
- *    `__DSH_MODULES__.import(spec)` — the seed-word branch, the one part of
- *    the module system that is stable across versions.
+ *    the injected module system's `import(spec)` (the `ctx.modules` service)
+ *    — the seed-word branch, the one part of the module system that is
+ *    stable across versions.
  *
  * Caching contract (three layers, each with a failure path):
  * - In-memory: one in-flight promise per chunk, memoized until
@@ -90,8 +91,7 @@ const CHUNK_REVALIDATE_TIMEOUT_MS = 5_000
  * The client module system surface this loader needs to resolve externals.
  * DSH 0.1.0-rc.8 provides it as the `ctx.modules` service (no page global
  * anymore); the plugin injects it at activation via
- * {@link setChunkModuleSystem}. The rc.7-era `window.__DSH_MODULES__` global
- * remains as a fallback so older hosts and the test harness keep working.
+ * {@link setChunkModuleSystem}.
  */
 export interface ChunkModuleSystem {
   import(specifier: string): Promise<unknown>
@@ -123,12 +123,11 @@ export function setChunkModuleSystem(system: ChunkModuleSystem | undefined): voi
 }
 
 /** Resolve the shell-installed module system (injected, then the plugin
- *  global shared with chunk-bundle copies, then the rc.7 page global). */
+ *  global shared with chunk-bundle copies). */
 function moduleSystem(): ChunkModuleSystem | undefined {
   const g = globalThis as Record<string, unknown>
   return injectedModuleSystem
     ?? g[MODULE_SYSTEM_GLOBAL] as ChunkModuleSystem | undefined
-    ?? (g as { __DSH_MODULES__?: ChunkModuleSystem }).__DSH_MODULES__
 }
 
 /** The plugin-owned chunk factory registry the chunk scripts populate. */