feat(editor): 拆分 EditorHost/TextEditor — code/markdown 成为注册 viewer，fs.read 增加 head 字节

- 新 src/client/EditorHost.tsx：'editor' tab 组件。matchFileViewer →
  按 fetchStrategy 分发（纯逻辑在 editor-load.ts）→ 渲染 viewer.component；
  binary 无匹配时渲染共享 BinaryDownload。标题栏保留在 host
- 新 src/client/TextEditor.tsx：code/markdown 的 viewer 组件（CodeMirror +
  保存/脏点/截断横幅 + md 预览/编辑切换），内容由 host 经 fsRead 传入，
  自身不再 fetch；按 viewerId === 'markdown' 分支（不再解析扩展名）
- 新 src/client/editor-load.ts：planFirstMatch / planFsReadOutcome /
  decodeHead 纯函数，策略分发可单测
- 新 src/client/binary-download.tsx：共享下载兜底组件（viewer 注册与
  host fallback 同一 UI）
- host src/index.ts：fs.read 二进制响应增加 head 字段（base64，前 4KB，
  唯一 host 触碰，设计文档 detect 机制所需）；客户端 FsBinaryResult 同步
- builtins：注册 8 个 viewer（新增 markdown exts md/markdown、code
  catch-all -100）；binary-download 增加 NUL detect + 复用 BinaryDownload
- 删除 src/client/EditorView.tsx（职责一分为二）；陈旧注释同步
- 新增 tests/editor-load.spec.ts（12 用例）

src/index.ts

@@ -123,8 +123,19 @@ async function resolveGitPath(cwd: string, raw: string): Promise<string> {
   return requireAbsolute(join(root, raw))
 }
 
-/** Text read of a file with the size cap; binary detection via NUL probe. */
-async function readText(path: string, readLimit: number): Promise<{ content: string; truncated: boolean; binary: boolean; size: number }> {
+/** How many leading bytes a binary read returns for client-side detect sniffing. */
+const READ_HEAD_LIMIT = 4096
+
+/** Text read of a file with the size cap; binary detection via NUL probe.
+ *  Binary reads also return the first {@link READ_HEAD_LIMIT} bytes (base64)
+ *  so the client can re-match viewers by content (`detect`). */
+async function readText(path: string, readLimit: number): Promise<{
+  content: string
+  truncated: boolean
+  binary: boolean
+  size: number
+  head?: string
+}> {
   const info = await stat(path).catch((error: unknown) => {
     throw new SidebarError('fs-error', `cannot read "${path}": ${error instanceof Error ? error.message : String(error)}`, 400)
   })
@@ -141,7 +152,10 @@ async function readText(path: string, readLimit: number): Promise<{ content: str
     const { bytesRead } = await handle.read(buffer, 0, buffer.length, 0)
     const slice = buffer.subarray(0, bytesRead)
     const binary = slice.includes(0)
-    return { content: binary ? '' : slice.toString('utf8'), truncated, binary, size }
+    const head = binary
+      ? slice.subarray(0, Math.min(slice.length, READ_HEAD_LIMIT)).toString('base64')
+      : undefined
+    return { content: binary ? '' : slice.toString('utf8'), truncated, binary, size, head }
   } finally {
     await handle.close()
   }
@@ -194,8 +208,8 @@ function buildApi(
       // Relative paths are git-derived (status/diff report repo-root-relative
       // names; the untracked diff view reads the file through this route).
       const path = await resolveGitPath(cwd, requireString(payload, 'path'))
-      const { content, truncated, binary, size } = await readText(path, resolved.readLimit)
-      if (binary) return { kind: 'binary', size, truncated }
+      const { content, truncated, binary, size, head } = await readText(path, resolved.readLimit)
+      if (binary) return { kind: 'binary', size, truncated, head }
       return { kind: 'text', content, truncated }
     },
     'fs.write': async (payload) => {