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/client/state.ts

@@ -11,7 +11,18 @@
  */
 import { SIDEBAR_PREFS_DEFAULTS, type SidebarPrefs } from '../prefs-shared.ts'
 
-export type TabType = 'explorer' | 'git' | 'editor' | 'terminal' | 'subagent' | 'diff'
+/**
+ * Tab type identifier. Builtins use the strings below; external plugins
+ * register their own ids (e.g. `'my-plugin:db'`) through the sidebar
+ * service. Kept as `string` so the registry is open; {@link SIDEBAR_BUILTIN_TAB_TYPES}
+ * lists the builtins for back-compat checks.
+ */
+export type TabType = string
+
+/** The tab type ids the plugin itself registers (see {@link ./builtins.ts}). */
+export const SIDEBAR_BUILTIN_TAB_TYPES = [
+  'explorer', 'git', 'editor', 'terminal', 'subagent', 'diff',
+] as readonly string[]
 
 /** What a diff tab shows: a worktree/index change of one path, or one commit's full patch. */
 export type SidebarDiffRef =
@@ -622,13 +633,10 @@ function sanitizeNode(node: unknown, seen: Set<string>, reid: Map<string, string
         droppedDiff = true
         continue
       }
-      if (
-        candidate.type !== 'explorer' && candidate.type !== 'git'
-        && candidate.type !== 'editor' && candidate.type !== 'terminal'
-        && candidate.type !== 'subagent'
-      ) {
-        return undefined
-      }
+      // Tab types are an open set (external plugins register their own);
+      // accept any string type here — an unregistered type renders an
+      // <OrphanedTab/> at view time and recovers if its plugin loads later.
+      if (typeof candidate.type !== 'string') return undefined
       tabs.push({
         id: candidate.id,
         type: candidate.type,