feat: a Reader's Sighting defers a Poll of a solitary Series (#103)

A userscript PUT already carries the Latest Chapter the Reader's own browser
read; it now stands in for a Poll where being wrong can hurt nobody else.
Deferral lives in the due query beside the rest cutoff: one Bookmark, sighted
within one rest, under the six-rest ceiling. checkOne judges the Reader whose
report raised the value off the comparison it already makes - three
contradictions stop them deferring, twenty confirmations forgive.

ADR-0011 records the trust model and the rejected alternatives.

backend/AGENTS.md

@@ -27,7 +27,8 @@ Guidance for OpenCode (and Claude Code) working under `backend/`. See root `AGEN
   `series` keyed `(site, series_id)`
   (`asura`|`demonic`|`comix`|`kagane`|`novelfull`|`lightnovelworld`) owns the
   shared facts — title, cover, canonical URL, `kind` (`manga`|`novel`),
-  Latest Chapter, `latest_checked_at` — and `bookmarks` holds only what
+  Latest Chapter, `latest_checked_at`, and the Sighting pair
+  `latest_sighted_at`/`latest_raised_by` (issue #103) — and `bookmarks` holds only what
   differs between readers: progress, favourite, lifecycle bucket,
   `updated_at`. A bookmark is keyed `(reader_id, site, series_id)` — no
   surrogate id; the wire `key` is derived as `site:series_id` on read — and
@@ -91,6 +92,23 @@ Guidance for OpenCode (and Claude Code) working under `backend/`. See root `AGEN
   every tick; found chapter written straight to the series row via
   `Store.SetLatestChapter`, so a bookmark's `updated_at` — and the list
   order — is never touched.
+  **Sightings** (issue #103, ADR-0011) let a Reader's own page read defer a
+  Poll: `Store.RecordSighting` — called by the PUT handler *before* the Upsert,
+  because the raise test needs the row as it stands — stamps
+  `series.latest_sighted_at` and, when the report raises the stored number,
+  names its Reader in `series.latest_raised_by`. The due query's HAVING clause
+  is where deferral lives: a Series is skipped only while it has exactly one
+  Bookmark, was sighted within one Rest, and is under the `sightingCeiling`
+  (six rests) since its last Poll. So a shared Series is never deferred, and no
+  Series goes six hours unpolled whatever arrives. `checkOne` judges the named
+  Reader off the comparison it already makes: a lower number is a
+  contradiction (logged with the Reader and both numbers), the same number an
+  agreement, a higher number the Site publishing and neither. Three
+  contradictions (`store.SightingDisagreementLimit`) stop that Reader deferring
+  — their reports still write the Latest Chapter — and twenty consecutive
+  agreements (`store.SightingAgreementsToClear`) forgive them, as does the
+  owner's clear-marks control. Deferral is recomputed from live facts every
+  round, so nothing needs invalidating when a Series gains a second Bookmark.
   Refusals and browser loss are Lane-local: two `errChallengeHeld` in one pass
   stop that Site for `refuseBackoff` (15m) while other Lanes continue; an
   `errBrowserInterrupted` (remote Chrome restart) sets a shared Poller flag