test(pins): widen whitelist for the episode-collections JSON shapes (item 2 series)

TestContractPin_CollectionsJSON tripped on the additive keys the
episode-collections work introduced — the tripwire worked exactly as
designed (additive keys must be whitelisted here AND in
docs/contract_pins.md first).

- Collections envelope gains rule_match_mode (item 2.4): the
  smart-collection match semantics on GET /api/collections/:id.
- BookInfo gains four STABLE keys (item 2.3): is_episode,
  feed_title, download_state, hidden — dropped omitempty so every
  card surface emits the same key set (the viewer_percentage
  precedent; adapters that predate episode awareness emit zero
  values).

docs/contract_pins.md changelog entry written. Nothing removed,
nothing renamed.

docs/contract_pins.md

@@ -468,3 +468,16 @@ never-queued episodes) is stamped from the `podcast_queue_seq` SEQUENCE
 at enqueue time so claim order = enqueue order (the effective listening
 order). Raw-row pins (TestContractPin_MediaListBothBranches →
 rawRowKeys) whitelist the key. Nothing removed, nothing renamed.
+
+## 2026-10-30 — episode collections (additive widening, plan item 2/2.3/2.4)
+
+Two shapes widen, both ADDITIVE — nothing removed, nothing renamed:
+
+- **Collections envelope** (`GET /api/collections/:id` → envelopeKeys)
+  gains `rule_match_mode` (smart-collection match semantics, 'any'
+  legacy default | 'all'). Static collections emit the default value.
+- **BookInfo** (every card surface → bookInfoKeys) gains four STABLE
+  keys — `is_episode`, `feed_title`, `download_state`, `hidden` — the
+  episode-collections extras. Adapters that predate episode awareness
+  emit their zero values (`false`/`""`), matching the
+  viewer_percentage precedent (nil-representable, key always present).