Chapter dates: read date setting and tiered release dates (BL-0047)

- Chapter rows show when you first finished a chapter (setting: date + hour,
  date, off) from a new durable progress doneAt; older reads fall back to the
  reading clock's earliest remembered completion.
- chapter.json keeps added (arrival in this library, now kept on replace) plus
  x-dates.source (source upload, bulk imports flagged), x-dates.release
  (official, for a later metadata job) and x-dates.ecosystem (first entry into
  any Manga Circus library; carried over peering, earliest wins, clamped).
- Rows show one release date by tier: official > source upload > ecosystem >
  library, with all known dates in the tooltip (setting to hide).
- Fixes: WeebCentral dates came from the chapter page, which has none (every
  chapter got the download time); they now come from the full chapter list.
  MangaDex no longer writes a group's publishAt into added. Peering sends dates.
- tools/backfill-chapter-dates.js migrates an existing library (dry run default).
- Coverage dry run on the server: docs/chapter-dates-coverage-2026-10-24.md.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

scraper/lib/publisher.js

@@ -12,6 +12,7 @@ const { directionFields } = require('../../reading-direction');
 const { mergeImportedMetadata } = require('./metadata-merge');
 const { normalizeWorkIdentity } = require('./work-identity');
 const { parseKey } = require('../../chapter-identity');
+const { publishedDates } = require('../../chapter-dates');
 
 const token = () => crypto.randomBytes(5).toString('hex');
 // Staging is a recovery asset, not disposable garbage. Legacy stages without an
@@ -180,7 +181,8 @@ async function stageChapter(bookDir, chapter, http, onPage, priorMeta = {}) {
     await fsp.writeFile(path.join(stage, 'chapter.json'), JSON.stringify({
       ...priorMeta,
       title: chapter.title || '', source: chapter.source,
-      added: chapter.added || new Date().toISOString(),
+      // "added" stays the first arrival in this library; x-dates keeps source/release/ecosystem.
+      ...publishedDates(priorMeta, { ...(chapter.dates || {}), added: chapter.added }),
       priority: Number(chapter.priority || 0), extra: !!chapter.extra,
       ...(chapter.special ? { special: true, label: chapter.label || 'Special', sourceId: chapter.sourceId,
         sourceNumber: chapter.sourceNumber ?? null,