Remove the client latest-chapter scan for lightnovelworld (#91) (#93)

Follows the spec published on #91: remove the client-side latest-chapter scan for lightnovelworld rather than porting #87's truncation into a second codebase.

- lightnovelworld.latestChapterFromAnchors deleted, not stubbed: absence is what the background-fetch guard keys off.
- computeLatestChapter tolerates an adapter with no scanner (yields null) and is exported as the test seam.
- backgroundRefreshLatest skips a scanner-less Site before the due filter: no Series page fetched, no freshness timestamp recorded, no batch slot consumed. The on-page path (maybeCaptureLatestOnSeriesPage) routes through the same null-tolerant computation.
- novelfull's scanner, the shared max-chapter helper and all four manga Sites untouched.
- userscript/AGENTS.md records the Poll-only contract for this Site and why.

All seven acceptance criteria from the spec met. node --check clean; novel suite 30/30 (the regression pin fails if a lnw scan is reintroduced, scoped or not); manga suite 35/35, manga userscript byte-for-byte unchanged.

Two-axis code review: no hard standard violations, spec-clean; one follow-up commit matching the sibling adapter guard from the manga script.

Reviewed-on: #93
Co-authored-by: Sulthan Zaki <sultankiki05@gmail.com>
Co-committed-by: Sulthan Zaki <sultankiki05@gmail.com>

userscript/AGENTS.md

@@ -78,6 +78,11 @@ Guidance for OpenCode (and Claude Code) working under `userscript/`. See root `A
   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.
+  The client performs no latest-chapter scan for this Site: the Poll's
+  one-hour cooldown dominates the client's four-hour throttle, so a scan
+  would add no freshness, and the page's wpdiscuz thread is a public write
+  surface a scan would have to truncate at. `computeLatestChapter` yields
+  null here and `backgroundRefreshLatest` skips the Site before any fetch.
 
 ### Second script: `novel-bookmark.user.js`