fix(library): seed-pass flip guard, manga null-sentinel, mangadex resolve

- comic/manga sync(): a note's first-ever observed issue/chapter number
  (no prior latest_issue/latest_chapter) now seeds the baseline without
  flipping read_status or notifying. Previously `prev === null` counted
  as "newer", so a Read note with no stored baseline flipped to Unread
  on its very first real sync. Flips now require a prior value AND a
  strictly greater candidate AND read_status Read, mirroring the
  watchlist build.ts `prevLast &&` guard.
- manga rss/last_chapter_date: quotedOrNull renders an unset value as
  the literal `null` token; on the next parse that string came back
  truthy, so an unset rss field was fetched as the URL "null" and
  isActive treated it as "has rss -> active" forever. Sentinel is now
  stripped back to '' everywhere it's read from frontmatter.
- manga resolve(): best-effort MangaDex id lookup by title (unique-exact
  match against attributes.title + altTitles) runs alongside the
  existing MAL lookup when the note has no mangadex_id yet. Any failure
  (no match, ambiguous, network error) is logged and skipped, leaving
  the mal_id patch intact.

tests/library-comic.test.ts

@@ -462,3 +462,26 @@ describe('comicSpec.sync — issue flip', () => {
 		expect(result!.content).toContain('latest_issue: 10');
 	});
 });
+
+describe('comicSpec.sync — seed pass (I3): no stored latest_issue never flips even when Read', () => {
+	test('Read comic, no stored latest_issue, API issue 10 -> seeds latest_issue, read_status stays Read, no notify', async () => {
+		const deps = makeDeps({ http: async () => comicvineFixture, getKey: () => 'cvkey' });
+		const fm = { comicvine_id: '195824', read_status: 'Read' };
+		const result = await comicSpec.sync(ctxFor(fm), deps);
+		expect(result!.flipped).toBe(false);
+		expect(result!.content).toContain('latest_issue: 10');
+		expect(result!.content).toContain('read_status: Read');
+		expect(deps.notifyCalls).toEqual([]);
+	});
+
+	test('subsequent sync with a higher issue number -> flip + notify (baseline now present)', async () => {
+		const cvNext = { results: { ...CV_RESULT, last_issue: { issue_number: '11', name: 'Next Issue' } } };
+		const deps = makeDeps({ http: async () => cvNext, getKey: () => 'cvkey' });
+		const fm = { comicvine_id: '195824', read_status: 'Read', latest_issue: '10' };
+		const result = await comicSpec.sync(ctxFor(fm), deps);
+		expect(result!.flipped).toBe(true);
+		expect(result!.content).toContain('latest_issue: 11');
+		expect(result!.content).toContain('read_status: Unread');
+		expect(deps.notifyCalls).toEqual(['«Absolute Batman» issue 11 out']);
+	});
+});