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/ui/components/VideoCard.js

@@ -2625,6 +2625,12 @@ export class VideoCard {
     if (this.video.pubkey) {
       this.root.dataset.ownerPubkey = this.video.pubkey;
     }
+    // Provenance flag for the liveness visibility policy: foreign/ingested videos
+    // (pulled in from other Nostr apps) can be hidden-until-verified separately
+    // from bitvid-native uploads. See cardSourceVisibility + CARD_LIVENESS_POLICY.
+    const isForeign =
+      this.video?.foreign === true || this.video?.source === "nip71-ingest";
+    this.root.dataset.foreign = isForeign ? "true" : "false";
   }
 
   applySourceDatasets() {