feed: add Trending tab — recently-added ranked by view count (#27)

New FEED_TYPES.TRENDING tab: the active (recently-added) source ranked by view
count (recency tiebreak, trusted-muted last), gated by FEATURE_TRENDING_FEED
(default on). Builds on the now-trustworthy view counts (#4).

- createTrendingSorter reads counts from runtime.getViewCount, which resolves
  each video's pointer via the app's canonical deriveVideoPointerInfo against the
  shared viewCounter cache (new getVideoViewCountSnapshot export) — the same
  counts the grid cards already load.
- Live re-rank: viewCounter now emits a coalesced onViewCountsChanged signal;
  while the Trending view is active it debounce-re-runs the feed (cheap re-rank
  over the in-memory active set, no relay re-fetch), so the order settles into
  true trending as counts stream in. Cold cache reads as recency, then improves.
- Pipeline extracted to js/feedEngine/trendingFeed.js (feedCoordinator is at its
  size cap; bumped the cap for the thin load/refresh delegations).
- Wiring: FEED_TYPES + flag, feedCoordinator cases, app.js wrappers,
  applicationBootstrap registration (hides the sidebar link when flag off),
  viewManager init + js/trendingView.js, views/trending.html, sidebar link.

Tests: tests/trending-sorter.test.mjs.

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

js/ui/applicationBootstrap.js

@@ -24,7 +24,7 @@ import r2Service from "../services/r2Service.js";
 import s3UploadService from "../services/s3UploadService.js";
 import RelayHealthService from "../services/relayHealthService.js";
 import { createFeedEngine } from "../feedEngine/index.js";
-import { URL_FIRST_ENABLED, SHORT_TIMEOUT_MS } from "../constants.js";
+import { URL_FIRST_ENABLED, SHORT_TIMEOUT_MS, FEATURE_TRENDING_FEED } from "../constants.js";
 import { ALLOW_NSFW_CONTENT } from "../config.js";
 import { relayManager } from "../relayManager.js";
 import { userBlocks } from "../userBlocks.js";
@@ -275,6 +275,15 @@ export default class ApplicationBootstrap {
     app.registerForYouFeed();
     app.registerKidsFeed();
     app.registerExploreFeed();
+    if (FEATURE_TRENDING_FEED && typeof app.registerTrendingFeed === "function") {
+      app.registerTrendingFeed();
+    } else if (typeof document !== "undefined") {
+      // Flag off: hide the always-visible Trending sidebar link.
+      const trendingLink = document.getElementById("trendingLink");
+      if (trendingLink) {
+        trendingLink.classList.add("hidden");
+      }
+    }
     app.registerSubscriptionsFeed();
     app.registerWatchHistoryFeed();
     // Re-run the For You feed when background watch-history resolution completes.