docs+fix: exports 加 ./client/api 别名；catch-all+detect 纯嗅探规则；设计文档偏差记录

- package.json exports 新增 './client/api' 别名指向 service.d.ts（设计文档
  §7 的公共导入路径，api.ts 已被 fetch 助手占用故不换文件名）
- matchFileViewer 补充 sniff-only 规则：exts: [] + detect 的 descriptor 在
  无 head 时不得盲认领（AGENTS.md 的 magic-sniffer 示例原实现会吞掉
  图片/PDF 等真实 viewer 的文件）；有 head 时只认领 detect 命中的。
  新增测试断言三种情形（无 head / magic 命中 / magic miss）
- docs/plans/2026-08-11-service-registry-design.md 追加 §17 实施偏差记录：
  dedupeKey+single 语法糖、available 三参、单趟匹配+嗅探规则、EditorHost/
  TextEditor 拆分、fs.read head 字段、导出别名、+ 菜单 disabled 行、
  保留的 store 级操作、内置清单、测试
- AGENTS.md：available 三参、single 语法糖、viewerId、单趟匹配+head 语义、
  8 viewer 内置清单、openTab seed.title 可选、参考实现指向 builtins/ 目录
- README 服务化章节：6 viewer → 8 viewer，标注全注册表驱动

src/client/service.ts

@@ -222,8 +222,20 @@ export function createBetterSidebarService(store: SidebarStore): BetterSidebarSe
     for (const v of Array.from(viewers.values()).sort(
       (a, b) => (b.priority ?? 0) - (a.priority ?? 0),
     )) {
-      if (head !== undefined && v.detect !== undefined && v.detect(path, head)) return v
-      if (v.exts.length === 0 || v.exts.includes(ext)) return v
+      // Content sniff first (only when head bytes are available).
+      if (head !== undefined && v.detect !== undefined) {
+        if (v.detect(path, head)) return v
+        // A catch-all with detect is SNIFF-ONLY: it must not blind-claim
+        // paths it never sniffed (a magic-number viewer must not swallow
+        // every file before the real viewers get their turn).
+        if (v.exts.length === 0) continue
+      } else if (v.exts.length === 0) {
+        // Blind catch-all (no detect) claims anything; a sniff-only
+        // catch-all (detect defined, no head yet) yields this round.
+        if (v.detect === undefined) return v
+        continue
+      }
+      if (v.exts.includes(ext)) return v
     }
     return undefined
   }