feat: 服务化改造 — ctx.betterSidebar 注册表（registerTab/registerFileViewer），内置 6 tab + 6 viewer 迁移到 registry，v0.4.0

- 新增 src/client/service.ts: BetterSidebarService 接口 + createBetterSidebarService 工厂
  - registerTab/registerFileViewer 返回 disposer（HMR-safe，挂到 cordis fiber）
  - matchFileViewer 三段匹配: priority desc → detect(内容嗅探) → exts
  - openTab 走 descriptor.dedupeKey 统一去重（替代 isSingle/editor-path/diff-id 三分支）
  - createTab 回调支持自定义 tab 创建（terminal 的 nextTerminal 递增）
- 新增 src/client/builtins.tsx: 6 内置 tab + 6 内置 viewer 自我注册（吃狗粮）
  - tab: editor(hidden) / explorer / git / subagent / terminal / diff(hidden)
  - viewer: image / pdf / docx / xlsx / pptx / binary-download
  - code/markdown 保持 EditorView 内联兜底（matchFileViewer 返回 undefined 时走 fsRead+CodeMirror）
- 新增 src/client/OrphanedTab.tsx: 未注册 type 的占位组件（插件未加载时降级）
- 改造 src/client/index.tsx: ctx.provide('betterSidebar') + registerBuiltins
- 改造 src/client/Sidebar.tsx: TabContent/buildNewTabOptions/onNewTab 查表替代 switch
- 改造 src/client/EditorView.tsx: 预览路由查 matchFileViewer 替代 if/else 链
- 改造 src/client/state.ts: TabType 联合字面量→string，sanitizeNode 接受所有 string type
- 改造 src/context-types.ts: declare module 'cordis' 加 betterSidebar 字段
- 改造 src/index.ts: re-export Context + 服务类型（consumer import type 触发 augmentation）
- package.json: 加 ./client/service exports 子路径，version 0.4.0
- tests/service.spec.ts: 12 个新测试覆盖 register/dispose/matchFileViewer/dedupe/createTab
- tests/unit.spec.ts: sanitizeState 测试更新（未知 type 不再被拒绝，渲染 OrphanedTab）

src/context-types.ts

@@ -20,6 +20,7 @@
 import type { IncomingMessage, ServerResponse } from 'node:http'
 import type { Duplex } from 'node:stream'
 import type { Context } from 'cordis'
+import type { BetterSidebarService } from './client/service.ts'
 
 /** One named webserver route (mirror of the host-webserver WebRoute). */
 export interface SidebarWebRoute {
@@ -309,6 +310,12 @@ declare module 'cordis' {
     settings: SidebarSettingsService
     invariants: SidebarInvariantsService
     tools: SidebarToolsService
+    /**
+     * The client-side sidebar registry: external plugins register tab types
+     * and file previewers here. Provided by the client half (see
+     * {@link ./client/index.tsx}); undefined on the host side.
+     */
+    betterSidebar: BetterSidebarService
     /**
      * Register a lifecycle callback (DSH-vendored cordis): runs at plugin
      * activation; its returned cleanup runs at disposal.