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/utils/cardSourceVisibility.js

@@ -1,3 +1,5 @@
+import { getCardLivenessPolicy } from "../constants.js";
+
 function resolveCardElement(cardLike) {
   if (!cardLike) {
     return null;
@@ -65,11 +67,22 @@ export function updateVideoCardSourceVisibility(cardLike) {
   const cdnHealthy = cdnState === "healthy";
   const streamHealthy = streamState === "healthy";
 
-  const shouldHide =
-    !cdnHealthy &&
-    !streamHealthy &&
-    !cdnPending &&
-    !streamPending;
+  // Liveness visibility policy (config/instance-config.js → CARD_LIVENESS_POLICY):
+  //   show-pending : show now, hide only once every source is confirmed dead.
+  //   hide-foreign : foreign/ingested cards stay hidden until a source is proven
+  //                  playable; native cards keep show-pending.
+  //   hide-all     : every non-owner card stays hidden until proven playable.
+  const policy = getCardLivenessPolicy();
+  const isForeign = card.dataset.foreign === "true";
+  const hideUntilVerified =
+    policy === "hide-all" || (policy === "hide-foreign" && isForeign);
+
+  const shouldHide = hideUntilVerified
+    ? // Hidden until at least one source proves playable (covers the pending
+      // window too, so a dead foreign stranger never flashes in).
+      !cdnHealthy && !streamHealthy
+    : // Default: only hide once every source has come back dead.
+      !cdnHealthy && !streamHealthy && !cdnPending && !streamPending;
 
   if (shouldHide) {
     if (!card.hidden) {