docs: add testing-the-userscript project skill

Documents the Node stub harness in userscript/test/logic.test.js: the four
globals it installs, why document.body is left undefined, the module.exports
test hook, and what is deliberately not testable (no DOM harness).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.claude/skills/testing-the-userscript/SKILL.md

@@ -0,0 +1,91 @@
+---
+name: testing-the-userscript
+description: Use when writing, running, or debugging tests for userscript/manga-bookmark.user.js — adding a case to logic.test.js, exporting a function for test, a test that fails with "is not a function"/undefined export, or deciding whether some userscript behaviour is testable at all.
+---
+
+# Testing the userscript
+
+`userscript/manga-bookmark.user.js` is a browser IIFE, not a module. It is tested
+by `require()`-ing it into Node under a hand-written four-object browser stub in
+`userscript/test/logic.test.js`. The harness covers **pure logic only** — adapters,
+parsers, helpers. UI, network, and storage behaviour are verified on-device.
+
+## Commands
+
+```bash
+node --check userscript/manga-bookmark.user.js      # parse check, silent on success
+node --test userscript/test/logic.test.js           # 14 tests as of 2026-07-28
+```
+
+Run both before every commit that touches the userscript.
+
+**Use the file path, not `node --test userscript/test/`.** The directory form
+fails `MODULE_NOT_FOUND` on this machine's Node v22.22.2. Older docs and plans
+still write the directory form — substitute the file path; do not try to fix it.
+
+## How the harness works
+
+The test file installs four globals **before** requiring the userscript:
+
+| Global | What it is | Why |
+|---|---|---|
+| `localStorage` | `Map`-backed stub | `loadCache`, `loadQueue`, and the key-migration IIFE touch it at module scope |
+| `location` | `{href, hostname, pathname, origin}` | read during boot |
+| `document` | `querySelector` for `meta[property="…"]` only, plus a no-op `addEventListener` | adapters read `og:title`/`og:image` |
+| `document.body` | **left `undefined`** | this is the whole trick |
+
+`document.body === undefined` sends the userscript's boot block down its `else`
+branch, where it waits for a `DOMContentLoaded` that never fires. `init()`,
+`buildUI()`, and every `fetch` stay dormant, so nothing else needs stubbing.
+
+The export hook near the end of the userscript is what makes `require()` work:
+
+```js
+if (typeof window === "undefined" && typeof module === "object" && module.exports) {
+  module.exports = { stripBuildHash, asura, demonic, anchorsFromHTML, statusOf };
+}
+```
+
+`typeof window === "undefined"` is load-bearing: under `@grant none` the script
+shares page globals, so it must not clobber a page's own UMD shim.
+
+## Adding a test
+
+1. If the function isn't already exported, add it to that `module.exports` list
+   and to the destructuring `require` at the top of `logic.test.js`. A test
+   failing with `X is not a function` means you skipped this step.
+2. Set `metaTags` (module-level `let` in the test file) for anything that reads
+   `og:` tags — it is reassigned per test, so set every tag your case needs.
+3. Build locations with the `loc(href)` helper; `detect()` reads only
+   `pathname`, `origin`, `href`, `hostname`.
+4. Keep the case pure: inputs in, value out, `assert` on the result.
+
+```js
+test("stripBuildHash removes a trailing 8-hex suffix", () => {
+  assert.equal(stripBuildHash("solo-leveling-059befe1"), "solo-leveling");
+});
+```
+
+## What is NOT testable here
+
+- **DOM, layout, Shadow DOM, the panel, the spinner.** There is no DOM harness
+  and **you must not add one** — no jsdom, no happy-dom, no second test file for
+  UI. Verified on-device (Cromite + Violentmonkey) instead.
+- **`fetch`, sync, the retry queue's network behaviour.** Verified against a
+  running backend.
+- Anything reachable only through `init()`/`buildUI()`.
+
+If a change's only meaningful verification is visual or on-device, say so in the
+report rather than inventing coverage.
+
+## Gotchas
+
+- **New module-scope code that touches a browser API breaks every test**, not
+  just a new one — the `require()` runs it. Keep such work inside functions that
+  only `init()` calls. If you must add module-scope access, extend the stub.
+- `stripBuildHash` must stay in sync with `asuraBuildHash` in `backend/store.go`;
+  changing one without the other silently splits series identity.
+- `document.querySelector` only understands `meta[property="…"]`. Any other
+  selector returns `null` — extend the stub rather than working around it.
+- The userscript stays GM-free (no `GM_*` APIs); a test that needs one is
+  testing something that can't ship.