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。

tests/chunk-loader.spec.ts

@@ -8,9 +8,10 @@
  * - externals resolve through the module system's seed branch (the stable,
  *   version-independent part), once per page,
  * - resetChunks drops the cache and the externals memo (HMR).
- * The production path runs against a fake `window.__DSH_MODULES__` and a
- * stub script loader that simulates the executed chunk script by assigning
- * the plugin-owned global factory registry.
+ * The production path runs against a fake module system injected via
+ * {@link setChunkModuleSystem} (mirroring the client half's `ctx.modules`
+ * injection) and a stub script loader that simulates the executed chunk
+ * script by assigning the plugin-owned global factory registry.
  */
 import { beforeEach, describe, expect, it, vi } from 'vitest'
 import './browser-globals.ts'
@@ -33,14 +34,10 @@ function installModuleSystem(): FakeModuleSystem {
   const fake: FakeModuleSystem = {
     import: vi.fn(async (specifier: string) => ({ seed: specifier })),
   }
-  ;(globalThis as Record<string, unknown>).__DSH_MODULES__ = fake
+  setChunkModuleSystem(fake)
   return fake
 }
 
-function removeModuleSystem(): void {
-  delete (globalThis as Record<string, unknown>).__DSH_MODULES__
-}
-
 /** Simulate a chunk script executing: it assigns its factory to the registry. */
 function simulateScript(name: string, factory: (require: (spec: string) => unknown) => ChunkExports): void {
   const g = globalThis as { __dshChunks__?: Record<string, unknown> }
@@ -49,7 +46,6 @@ function simulateScript(name: string, factory: (require: (spec: string) => unkno
 }
 
 beforeEach(() => {
-  removeModuleSystem()
   setChunkModuleSystem(undefined)
   delete (globalThis as Record<string, unknown>).__dshChunks__
   resetChunks()
@@ -89,9 +85,6 @@ describe('test-registry path (vitest / jsdom-less environments)', () => {
 describe('production path (script injection + global registry + externals require)', () => {
   it('resolves externals through an injected ctx.modules system (rc.8 — no page global)', async () => {
     const modules = installModuleSystem()
-    // rc.8 drops window.__DSH_MODULES__; the client half injects ctx.modules.
-    removeModuleSystem()
-    setChunkModuleSystem(modules)
     const loaded: string[] = []
     setChunkScriptLoaderForTests(async (src) => {
       loaded.push(src)