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.

docs/adr/0011-sighting-deferral-trust-model.md

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+# ADR-0011: Sightings — a Reader report defers a Poll where being wrong hurts only them
+
+Date: 2026-08-16
+Status: accepted
+
+## Decision
+
+A **Sighting** is what a userscript PUT already carries: the Latest Chapter the
+Reader's own browser read off the Series page. It is now allowed to stand in for
+a Poll, under one restriction and one ceiling:
+
+- **Solitary Series only.** A Sighting defers the Poll of a Series exactly one
+  Bookmark points at. A Series two Readers share is Polled on schedule no matter
+  how recently it was sighted.
+- **One rest of standing.** A Sighting postpones Polls for one Rest
+  (`defaultRest`, an hour), not forever: a Series nobody visits again returns to
+  the normal schedule by itself.
+- **Six-hour ceiling.** `sightingCeiling = 6 * defaultRest`. However many
+  Sightings arrive, a Series unpolled for six hours is Polled.
+
+Both live in the due query's HAVING clause (`store.DueForLatestCheck`), beside
+the Rest cutoff — the same place the schedule has always been decided, so no
+timer and no second code path can disagree with it.
+
+Attribution and judgement:
+
+- `Store.RecordSighting` runs *before* the Upsert that stores the reported
+  value, because the raise test needs the row as it stands. A report that raises
+  the stored Latest Chapter names its Reader in `series.latest_raised_by`.
+- The Poll is the oracle. `Poller.checkOne` already compares what the Site
+  publishes against what is stored, so judgement costs no extra request: a lower
+  number contradicts the Sighting (`sighting_disagreements + 1`, both numbers and
+  the Reader logged), the same number confirms it (`sighting_agreements + 1`), a
+  **higher** number is the Site publishing and means nothing either way.
+- At `SightingDisagreementLimit` (3) that Reader's Sightings stop deferring
+  anything. They still write the Latest Chapter — the penalty removes a
+  privilege, it does not silence anyone.
+- `SightingAgreementsToClear` (20) consecutive confirmations forgive the
+  disagreements. A disagreement resets the run to zero.
+- The owner clears marks from the administration page (issue #102, shipped
+  first precisely so a false mark has a remedy the day the mechanism lands).
+
+## Why
+
+Most of the backend's work was redundant. The userscript reads the Latest
+Chapter on every Series page visit and PUTs it; minutes later the Poll Lane
+fetches the same page for the same number. Deferring on a report converts a
+visit into a Poll saved, which is Lane capacity handed back to Series nobody is
+reading.
+
+The restriction is the whole safety argument, and it is about **blast radius**,
+not about trust arithmetic:
+
+- On a solitary Series, a wrong report can only mislead the Reader who made it.
+  There is nobody else's ember to falsify.
+- On a shared Series it could mislead someone else, so a report never postpones
+  anything there.
+
+The ceiling bounds the damage in time: a false value dies within six hours
+whatever happens, because the Poll that finds it is guaranteed. That is also
+what makes lying pointless — the six-hour audit is certain, not sampled, so a
+determined attacker buys at most three ceilings' worth of a wrong number on
+their own Series and then loses deferral entirely.
+
+The cost of recovery is deliberate. An agreement is only recorded when a later
+Poll confirms a Sighting, so twenty agreements are twenty Polls of Series that
+Reader bookmarks — hours to days of real time, not twenty page views. Waiting is
+therefore not a strategy, and credit cannot be banked in advance.
+
+## Tradeoffs and rejections
+
+- **Trusting a Sighting on a shared Series** rejected: it is the only case where
+  one Reader's mistake reaches another Reader's list, and no amount of
+  reputation makes that recoverable within the six-hour window.
+- **Cross-Reader agreement, voting, weighting, consensus scoring** rejected: a
+  single-source report cannot have its confidence evaluated by comparison, and
+  with the typical two Readers a disagreement identifies nothing. The Poll is
+  the only oracle in the system, so it is the only judge.
+- **A randomised audit** (Poll a fraction of deferred Series) rejected in favour
+  of the fixed ceiling: a sampled audit makes the attacker's expected cost a
+  probability, while a ceiling makes it a certainty, and a certainty is what
+  makes the solitary-Series rule defensible in one sentence.
+- **Blocking a marked Reader's writes** rejected: the Latest Chapter they report
+  is still the best available value, and their Sightings must keep being judged
+  or they could never earn the privilege back.
+- **Per-Series flagging** rejected in favour of per-Reader marks: a Series is
+  not the thing that can be wrong. Naming the Reader and logging both numbers is
+  also what distinguishes a broken Site adapter (every Reader of that Site
+  contradicted at once) from one bad actor.
+- **Timers or a background reputation job** rejected: deferral is decided from
+  live facts every round — Bookmark count, sighting timestamp, the Reader's
+  marks — so a Series that gains a second Bookmark stops deferring at once, with
+  nothing to invalidate.
+
+## Constraints preserved
+
+- A Sighting is not Progress: it may move the Latest Chapter and nothing else.
+  `updated_at` never moves, so a report cannot reorder the list (ADR-0004).
+- The Latest Chapter is a Series-level fact (ADR-0003): a Sighting writes the
+  shared row, so every Reader of a shared Series sees it immediately — deferral
+  is the only thing the solitary rule withholds.
+- Ember means new chapter only (`docs/design-system.md`): a marked Reader
+  renders no differently in their own list, and nothing about the trust model
+  reaches the Series list's colour.
+- The Poll remains authoritative. Where a Sighting and a Poll disagree, the
+  Poll's value is what gets stored.