Read lnw Series identity from the chapter page pointer (#89)

The lightnovelworld chapter branch no longer derives the Series address by
string-manipulating the chapter path: on ~7% of novels the Chapter Slug
diverges from the Series slug and the derived address 404s on every Poll.
The identity now comes from the page's own a[aria-label='All Chapter']
pointer (breadcrumb's second crumb as fallback), validated as a
lightnovelworld.net /novel/<slug>/ address; no pointer resolves to
type: other. The Chapter Slug rides on the detected page object only
(chapterSlug; null on a series page) and is written to no store, for the
stale-row repair in #90.

Harness: document.querySelector now answers attribute selectors with an
element exposing getAttribute (attrEls table, cleared by reset()).
AGENTS.md records that the Series address is discovered, not derived.

userscript/AGENTS.md

@@ -70,9 +70,13 @@ Guidance for OpenCode (and Claude Code) working under `userscript/`. See root `A
   Behind a Cloudflare JS challenge no TLS fingerprint
   clears, so the backend polls it through the headless browser.
 - **lightnovelworld.net** (novel script): series `/novel/<slug>/`, chapter
-  `/<slug>-chapter-<n>/` — flat, at the site root. `h1.entry-title` is the clean
-  title on a series page and `<Title> Chapter <n>` on a chapter page. Its series
-  page lists every chapter with an
+  `/<slug>-chapter-<n>/` — flat, at the site root. The chapter path's slug is a
+  Chapter Slug, not an identity: the Series address is read off the page's
+  `a[aria-label='All Chapter']` (fallback: the BreadcrumbList's second crumb),
+  and a Series may publish under several Chapter Slugs. A chapter page with no
+  pointer resolves to `other`, so no Bookmark is offered. `h1.entry-title` is
+  the clean title on a series page and `<Title> Chapter <n>` on a chapter page.
+  Its series page lists every chapter with an
   absolute href, so the backend polls it with the plain TLS client.
 
 ### Second script: `novel-bookmark.user.js`