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/editor-load.ts

@@ -0,0 +1,92 @@
+/**
+ * Pure editor-load planning: the decision logic the editor host needs to
+ * turn a matched file viewer + a host fs.read result into a render action.
+ * Kept dependency-free (no React, no fetch) so the strategy dispatch is
+ * unit-testable and the wire contract (head bytes, binary flag) is pinned.
+ *
+ * The host flow this module drives:
+ *   1. `matchFileViewer(path)` picks a viewer by extension/priority.
+ *   2. `planFirstMatch` dispatches its fetchStrategy.
+ *   3. An fsRead viewer fetches through the host; `planFsReadOutcome`
+ *      decides what to do with the result — including the head-based
+ *      re-match that lets a `detect` viewer claim a binary the extension
+ *      match could not see (the builtin NUL probe on `binary-download`).
+ */
+import type { FileViewerDescriptor } from './service.ts'
+
+/** One host fs.read result (mirror of the wire; `head` present when binary). */
+export interface FsReadOutcome {
+  binary: boolean
+  content: string
+  truncated: boolean
+  /** base64 of the first bytes (present on binary reads; sniffing material). */
+  head?: string
+}
+
+/** What the editor host should do next. */
+export type EditorLoadAction =
+  /** No renderer: show the download UI. */
+  | { kind: 'binary' }
+  /** Render `viewer`'s component with the carried payload. */
+  | { kind: 'render'; viewer: FileViewerDescriptor; content?: string; truncated?: boolean; mediaUrl?: string; customData?: unknown }
+  /** Fetch the file through the host (fsRead strategy). */
+  | { kind: 'fetchFsRead'; viewer: FileViewerDescriptor }
+  /** Call the viewer's load() and render with its return value. */
+  | { kind: 'customLoad'; viewer: FileViewerDescriptor }
+
+/** Decode the host's base64 head bytes into the sniffing buffer. */
+export function decodeHead(headBase64: string): Uint8Array {
+  const binary = atob(headBase64)
+  const bytes = new Uint8Array(binary.length)
+  for (let i = 0; i < binary.length; i += 1) bytes[i] = binary.charCodeAt(i)
+  return bytes
+}
+
+/**
+ * Dispatch one matched viewer's fetchStrategy. A missing viewer or a
+ * `binary-download` strategy both mean "no client-side renderer" → the
+ * download UI. `mediaUrlOf` builds the media URL for `mediaUrl`/`none`
+ * strategies (pure, but scope-bound — injected by the host).
+ */
+export function planFirstMatch(
+  viewer: FileViewerDescriptor | undefined,
+  mediaUrlOf: () => string,
+): EditorLoadAction {
+  if (viewer === undefined || viewer.fetchStrategy === 'binary-download') return { kind: 'binary' }
+  switch (viewer.fetchStrategy) {
+    case 'mediaUrl':
+    case 'none':
+      return { kind: 'render', viewer, mediaUrl: mediaUrlOf() }
+    case 'custom':
+      return { kind: 'customLoad', viewer }
+    case 'fsRead':
+      return { kind: 'fetchFsRead', viewer }
+  }
+}
+
+/**
+ * Decide what an fsRead result means for the editor.
+ * - Text: the first match stands (content is valid for any fsRead viewer).
+ * - Binary: the host head bytes enable a re-match — a `detect` viewer (e.g.
+ *   a plugin sniffing a binary format) may claim the file. `custom` viewers
+ *   load their own bytes; `mediaUrl`/`none` viewers render the media route;
+ *   an fsRead viewer or nothing cannot render binary → download UI.
+ */
+export function planFsReadOutcome(
+  viewer: FileViewerDescriptor,
+  result: FsReadOutcome,
+  rematch: (head: Uint8Array) => FileViewerDescriptor | undefined,
+  mediaUrlOf: () => string,
+): EditorLoadAction {
+  if (!result.binary) {
+    return { kind: 'render', viewer, content: result.content, truncated: result.truncated }
+  }
+  const claimed = result.head === undefined ? undefined : rematch(decodeHead(result.head))
+  if (claimed !== undefined && claimed.fetchStrategy === 'custom') {
+    return { kind: 'customLoad', viewer: claimed }
+  }
+  if (claimed !== undefined && (claimed.fetchStrategy === 'mediaUrl' || claimed.fetchStrategy === 'none')) {
+    return { kind: 'render', viewer: claimed, mediaUrl: mediaUrlOf() }
+  }
+  return { kind: 'binary' }
+}