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/internal/latest/poller.go

@@ -250,7 +250,8 @@ func (p *Poller) runLanePass(ctx context.Context, name string, paced bool) time.
 		return defaultGap
 	}
 
-	due, err := p.Store.DueForLatestCheck(name, now.Add(-s.Rest).UnixMilli())
+	due, err := p.Store.DueForLatestCheck(name,
+		now.Add(-s.Rest).UnixMilli(), now.Add(-sightingCeiling).UnixMilli())
 	if err != nil {
 		log.Printf("latest poll %s: due query: %v", name, err)
 		st.Gap = defaultGap
@@ -461,6 +462,11 @@ func (p *Poller) checkOne(ctx context.Context, sr store.Series) error {
 		return nil
 	}
 
+	// The Poll is the oracle for whatever Sighting last raised this Series
+	// (issue #103), and the judgement is free: the comparison below already
+	// exists, and no extra request is made to reach it.
+	p.judgeSighting(sr, facts.Latest.Num)
+
 	// Equality, not >, mirroring the userscript (L427): a site that retracts a
 	// chapter should correct the stored number downward. The comparison is
 	// against the due-query snapshot; a concurrent write in between only costs
@@ -481,6 +487,36 @@ func (p *Poller) checkOne(ctx context.Context, sr store.Series) error {
 	return nil
 }
 
+// judgeSighting settles the Sighting the Series' stored Latest Chapter is owed
+// to, if any, against what the Site actually publishes. The asymmetry is the
+// whole of the detection rule and is what keeps it free of false alarms: a Poll
+// finding a *lower* number than stored means the Reader who raised it reported
+// a chapter that does not exist, while a Poll finding a higher one is only the
+// Site publishing since and means nothing about the report. Equality confirms
+// the report, which is how an honest Reader earns back a mark.
+//
+// A Series with no attribution — the stored value is a Poll's own, or a
+// previous Poll already judged the report — is nobody's to answer for.
+func (p *Poller) judgeSighting(sr store.Series, found float64) {
+	if sr.LatestRaisedBy == nil || sr.LatestChapterNum == nil {
+		return
+	}
+	stored := *sr.LatestChapterNum
+	if found > stored {
+		return
+	}
+	if found < stored {
+		// Logged with both numbers and the Reader, because that is what tells a
+		// broken Site adapter (which marks every Reader of that Site at once)
+		// from one Reader deliberately lying.
+		log.Printf("latest poll %q: sighting contradicted: reader %d raised it to %v, site publishes %v",
+			sr.Key(), *sr.LatestRaisedBy, stored, found)
+	}
+	if err := p.Store.RecordSightingOutcome(sr.Site, sr.SeriesID, *sr.LatestRaisedBy, found == stored); err != nil {
+		log.Printf("latest poll %q: record sighting outcome: %v", sr.Key(), err)
+	}
+}
+
 // healCover runs prefetchCover in the background. Cover bytes come from a
 // different host — often a CDN — and heal once in a Series's life, so they
 // must not consume a Lane's gap: a large import with many blanks would