feat(liveness): config-driven card visibility policy + probe prefetch (#5)

Makes the card liveness-visibility behavior switchable so it can be A/B
feel-tested, and probes ahead of the scroll for a snappier result.

- CARD_LIVENESS_POLICY (config/instance-config.js): "show-pending" (default,
  unchanged) | "hide-foreign" (foreign/ingested cards hidden until a source
  verifies; native cards stay show-pending) | "hide-all". Owner's own cards are
  always visible. Flip live for testing without a rebuild:
  window.__BITVID_CARD_LIVENESS_POLICY__ = "hide-foreign" then refresh/scroll.
  Implemented via a data-foreign provenance flag on the card +
  getCardLivenessPolicy() in cardSourceVisibility.js.
- LIVENESS_PROBE_PREFETCH_MARGIN (default 600px): both liveness probes were
  already viewport-gated (IntersectionObserver, on-screen-first, WebTorrent
  priority-queued + concurrency-capped); this expands their rootMargin so cards
  just below the fold verify before the user reaches them.
- Tests: three policy scenarios in tests/video-card-source-visibility.test.mjs;
  also un-broke 3 pre-existing VideoCard tests by adding the missing
  IntersectionObserver JSDOM stub.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

docs/video-liveness-plan.md

@@ -57,11 +57,14 @@ but a WebTorrent source is live, keep the card.
   concurrency (it's cheap vs WebTorrent swarm joins).
 - WebTorrent `PROBE_TIMEOUT_MS` (20s): keep generous (swarm joins are slow) but
   ensure it runs in parallel and never blocks the CDN verdict.
-- DECISION NEEDED: **show-pending (current) vs hide-until-verified.** Showing
-  pending means a brief flash of soon-to-be-hidden dead cards; hide-until-verified
-  means good cards pop in after the probe. Recommend: keep show-pending for cards
-  with a *bitvid-native/whitelisted* source (fast/trusted), but **hide-until-verified
-  for foreign/ingested** videos (so unplayable strangers never flash in).
+- DECISION (2026-06-24): made **config-driven for live A/B testing** instead of
+  hard-coding one policy. `CARD_LIVENESS_POLICY` =
+  `show-pending` (default) | `hide-foreign` | `hide-all`, flippable live via
+  `window.__BITVID_CARD_LIVENESS_POLICY__`. Recommendation stands: `hide-foreign`
+  (native = show-pending, foreign/ingested = hidden-until-verified). Also added a
+  configurable probe prefetch margin (`LIVENESS_PROBE_PREFETCH_MARGIN`, default
+  600px) so cards verify before they scroll into view. Pick the final default
+  after feel-testing.
 
 ## Reuse map
 - `js/ui/urlHealthController.js` (probeUrl / probeUrlWithVideoElement) — make