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>

graphify-out/manifest.json

@@ -395,13 +395,13 @@
     "semantic_hash": "8008a187690764436540fab47ba0cfcc"
   },
   "userscript/novel-bookmark.user.js": {
-    "mtime": 1786446006.925938,
-    "ast_hash": "64f8d2848294c7aeb8b2e9d43a049fd0",
+    "mtime": 1786454823.798836,
+    "ast_hash": "834effb0821f8d6c9f57f6554a5db462",
     "semantic_hash": ""
   },
   "userscript/test/novel-logic.test.js": {
-    "mtime": 1786446006.925938,
-    "ast_hash": "be732600e57fe4ad2e585e9c84d6f341",
+    "mtime": 1786454566.070801,
+    "ast_hash": "b25a7377af210dd0aff8284501fd0252",
     "semantic_hash": ""
   },
   ".opencode/agent/implementer.md": {
@@ -510,13 +510,13 @@
     "semantic_hash": "69114d07ed792d6bb1d13758ba5435e1"
   },
   "userscript/AGENTS.md": {
-    "mtime": 1786446006.9219902,
-    "ast_hash": "954caa07b01ab323ab205e29f0bcacd8",
+    "mtime": 1786454605.6727684,
+    "ast_hash": "e276ffe9a6e7b55fd3235466a1995c22",
     "semantic_hash": ""
   },
   "userscript/CLAUDE.md": {
-    "mtime": 1786446006.9219902,
-    "ast_hash": "954caa07b01ab323ab205e29f0bcacd8",
+    "mtime": 1786454605.6727684,
+    "ast_hash": "e276ffe9a6e7b55fd3235466a1995c22",
     "semantic_hash": ""
   },
   "backend/internal/web/static/login-art.png": {