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]>

js/urlHealthObserver.js

@@ -1,8 +1,11 @@
 import { createCardObserver } from "./dom/cardObserver.js";
 import { safeDecodeURIComponent } from "./utils/safeDecode.js";
 import { userLogger } from "./utils/logger.js";
+import { getLivenessProbePrefetchMargin } from "./constants.js";
 
-const ROOT_MARGIN = "0px";
+// Prefetch margin lets cards just below the fold start probing before they scroll
+// into view, so they're verified by the time the user reaches them.
+const ROOT_MARGIN = getLivenessProbePrefetchMargin();
 const THRESHOLD = 0.25;
 
 const urlCardObserver = createCardObserver({