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>

docs/library-format.md

@@ -208,11 +208,39 @@ An AniList refresh with a different ID is stopped for review before chapter publ
 |-------|------|---------|
 | `title` | string | Chapter title without "Chapter N" (default: title part of the folder name) |
 | `source` | `{name, url}` or string | Where this version came from (default: `[source]` in the folder name) |
-| `added` | ISO date/time string | Release (or download) date, used for "recently updated" (default: the folder's modification time) |
+| `added` | ISO date/time string | When this version entered **this** library, used for "recently updated" (default: the folder's modification time). The scraper keeps it when it replaces a version |
+| `x-dates` | object | Other chapter dates, written by the scraper (see below) |
 | `extra` | boolean | Marks the chapter as extra without moving it |
 | `priority` | number | Tie-breaker between versions with equal votes. Higher wins (default `0`) |
 | `direction` | as above | Overrides book/manga direction, e.g. a colour webtoon special inside a printed series |
 
+#### Chapter dates (`x-dates`)
+
+```json
+"added": "2026-10-17T15:04:03.424Z",
+"x-dates": {
+  "version": 1,
+  "source": { "at": "2026-10-18T15:06:05.348Z", "name": "WeebCentral", "batch": false },
+  "release": { "at": "2023-09-17", "precision": "day", "source": "MangaUpdates · Viz" },
+  "ecosystem": { "at": "2026-10-17T15:04:03.424Z", "via": "this server" }
+}
+```
+
+- `source`: when the source site uploaded this version (WeebCentral's full chapter list,
+  MangaDex `publishAt`). `batch: true` means it arrived with 3 or more other uploads
+  less than 10 minutes apart, which is a site import, not a release.
+- `release`: the official public release (`precision` `day`, `month` or `year`). Not
+  filled yet; a metadata job will add it.
+- `ecosystem`: when the chapter first entered any Manga Circus library. Peer transfers
+  carry it, and the earliest known date always wins. A peer's claim can't be earlier than
+  the source upload or later than now.
+
+The chapter list shows one release date per chapter, choosing the first tier that has
+a date: `release` → `source` unless `batch` (shown as ≈) → `ecosystem` (◎) → `added` (↓).
+Each tier takes the earliest value across the chapter's versions. Existing libraries get
+these fields from `node tools/backfill-chapter-dates.js <library>`: a dry run by default,
+then `--apply --writers-stopped` and a deep Rescan.
+
 Manual directions have priority at **chapter → book → manga**. A `directionOverride`
 or a plain `direction` without matching generated provenance is manual. Otherwise,
 the scanner checks the version's existing image dimensions for coherent tall strips