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/sites.go

@@ -409,6 +409,13 @@ const (
 	// asleep (ADR-0005 on-demand browser).
 	browserWakeCount = 5
 	browserWakeAge   = 15 * time.Minute
+	// sightingCeiling caps Sighting deferral (issue #103): however many
+	// Sightings arrive, a Series unpolled for this long is Polled. It is what
+	// makes a client report safe to trust — a wrong Latest Chapter dies within
+	// the ceiling deterministically, rather than in expectation the way a
+	// randomised audit would have it. Six rests, so a Series a Reader visits
+	// constantly still gets one authoritative check per working day-part.
+	sightingCeiling = 6 * defaultRest
 )
 
 // effectiveGap is a Site's pace: the registry gap, or one rest divided by the