feat(feed): make For You inclusive + ranked by your lists (G1-G5)

The For You feed was chronological, hashtag-interest-EXCLUSIVE, and ignored
follows entirely (docs/feed-algo-audit.md). Now it is inclusive + ranked:

- createForYouScorerStage scores each video: follows-boost (author you follow)
  + interest/watch-topic affinity (cosine vs your interests+watch-history vector)
  + freshness - disinterest penalty. Reuses Explore's vector/similarity helpers.
- createForYouScoreSorter ranks by that score (muted last, recency tiebreak).
- For You pipeline: interests now BOOST not exclude (enforceInterests:false);
  scorer added; chronological sorter replaced. Never empty (falls back to
  recency when you have no follows/interests/history).
- buildForYouFeedRuntime now feeds subscriptionAuthors + watchHistoryTagCounts.

Watched/blocked/muted are still removed (suppression/blacklist/moderation
stages unchanged). Cheat-resistant tests (older boosted items only win via the
boost). Grid renders end-to-end (~1.3s). Weight tuning is a follow-up.

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

js/feedEngine/index.js

@@ -10,11 +10,12 @@ export {
   createModerationStage,
   createResolvePostedAtStage,
 } from "./stages.js";
-export { createExploreScorerStage } from "./exploreScoring.js";
+export { createExploreScorerStage, createForYouScorerStage } from "./exploreScoring.js";
 export { createKidsScorerStage } from "./kidsScoring.js";
 export { createKidsAudienceFilterStage } from "./kidsAudienceFilterStage.js";
 export {
   createChronologicalSorter,
+  createForYouScoreSorter,
   createExploreDiversitySorter,
   createKidsScoreSorter,
 } from "./sorters.js";