docs: record the stale-news / clientRank fix as a key decision

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

CLAUDE.md

@@ -120,6 +120,18 @@ API/contract.md               — REST + WebSocket API spec
 - Pagination: cursor-based via `nextCursor`.
 - `StoryStore.setPill()` resets `lastSyncedAt` to bypass the 30s rate-limit on filter switch.
 - `sectionSupplement` fetches populate sparse regional sections in the All digest independently.
+- **Age must be read from `firstSeenAt`, never `updatedAt`.** The backend bumps `updatedAt`
+  every time it attaches another article to a cluster, however loosely related — a story
+  can read "1 hr ago" while being weeks old (see `docs/BACKLOG.md` #15). `firstSeenAt` is
+  the honest age and is already decoded on `StorySummary`.
+- **Stale-news control (client-side, `Views/Shared/Theme.swift`):** `clientRank` applies a
+  soft time-decay to `signalScore` so fresh stories outrank stale high-scorers in
+  `feedOrder` — it computes age from `firstSeenAt`, not `updatedAt` (was silently broken
+  by the same bug above until fixed). `StorySummary.isFossil` (`firstSeenAt` > 5 days) is a
+  hard backstop on top of that: `SignalFeedView.mainStories` and `.makeDigest()` sink
+  fossils below current stories so one can never take the hero/lead slot, regardless of
+  score. Fossils still appear in the feed — just not in the most prominent position. This
+  is a display/ordering patch; the server's own ranking is still wrong at the source.
 
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