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/client/EditorHost.tsx

@@ -0,0 +1,101 @@
+/**
+ * The editor tab host: resolves a file's previewer through the sidebar
+ * registry (`matchFileViewer`), fetches bytes per the matched viewer's
+ * fetch strategy, and renders its component — or the shared download pane
+ * when nothing can render the file. The header shows the file title; the
+ * editable code/markdown viewers render their own toolbar below it.
+ *
+ * The strategy dispatch is pure (planFirstMatch / planFsReadOutcome in
+ * editor-load.ts); this component only wires it to the host APIs.
+ */
+import { useEffect, useState } from 'react'
+import { createElement } from 'react'
+import type { Context } from '../context-types.ts'
+import { api, mediaUrl, type SessionScope } from './api.ts'
+import { BinaryDownload } from './binary-download.tsx'
+import { planFirstMatch, planFsReadOutcome, type EditorLoadAction } from './editor-load.ts'
+import { t } from './locales.ts'
+import type { FileViewerDescriptor } from './service.ts'
+import type { SidebarStore } from './state.ts'
+import css from './sidebar.module.css'
+
+type EditorLoad =
+  | { status: 'loading' }
+  | { status: 'error'; message: string }
+  | { status: 'ready'; viewer: FileViewerDescriptor; content?: string; truncated?: boolean; mediaUrl?: string; customData?: unknown }
+  | { status: 'binary' }
+
+export function EditorHost(props: { ctx: Context; store: SidebarStore; scope: SessionScope; path: string; title: string }) {
+  const { ctx, store, scope, path, title } = props
+  const [load, setLoad] = useState<EditorLoad>({ status: 'loading' })
+
+  useEffect(() => {
+    let cancelled = false
+    setLoad({ status: 'loading' })
+    const mediaUrlOf = (): string => mediaUrl(scope, path)
+    const apply = (action: EditorLoadAction): void => {
+      if (cancelled) return
+      switch (action.kind) {
+        case 'binary':
+          setLoad({ status: 'binary' })
+          return
+        case 'render':
+          setLoad({
+            status: 'ready',
+            viewer: action.viewer,
+            content: action.content,
+            truncated: action.truncated,
+            mediaUrl: action.mediaUrl,
+            customData: action.customData,
+          })
+          return
+        case 'customLoad':
+          void action.viewer.load?.(path, scope).then((data) => {
+            if (cancelled) return
+            setLoad({ status: 'ready', viewer: action.viewer, customData: data })
+          }).catch((error: unknown) => {
+            if (cancelled) return
+            setLoad({ status: 'error', message: error instanceof Error ? error.message : String(error) })
+          })
+          return
+        case 'fetchFsRead':
+          api.fsRead(scope, path).then((result) => {
+            if (cancelled) return
+            // Binary reads carry the head bytes for the detect re-match.
+            const outcome = planFsReadOutcome(action.viewer, {
+              binary: result.kind === 'binary',
+              content: result.kind === 'text' ? result.content : '',
+              truncated: result.truncated,
+              head: result.kind === 'binary' ? result.head : undefined,
+            }, (head) => ctx.betterSidebar?.matchFileViewer(path, head), mediaUrlOf)
+            apply(outcome)
+          }).catch((error: unknown) => {
+            if (cancelled) return
+            setLoad({ status: 'error', message: error instanceof Error ? error.message : String(error) })
+          })
+          return
+      }
+    }
+    apply(planFirstMatch(ctx.betterSidebar?.matchFileViewer(path), mediaUrlOf))
+    return () => { cancelled = true }
+  }, [scope.sessionId, scope.cwd, path, ctx])
+
+  return (
+    <div className={css.editor}>
+      <div className={css.editorHeader}>
+        <span className={css.editorTitle} title={path}>{title}</span>
+      </div>
+      {load.status === 'loading' && <div className={css.editorPlaceholder}>{t('loading')}</div>}
+      {load.status === 'error' && <div className={css.editorError}>{load.message}</div>}
+      {load.status === 'binary' && <BinaryDownload scope={scope} path={path} />}
+      {load.status === 'ready' && createElement(load.viewer.component, {
+        ctx, store, scope, path, title,
+        viewerId: load.viewer.id,
+        content: load.content,
+        truncated: load.truncated,
+        mediaUrl: load.mediaUrl,
+        customData: load.customData,
+      })}
+    </div>
+  )
+}