feed: make Explore "new-to-you + diverse, less recency"

Explore shared the full active set with Recent/For You and leaned on freshness,
so it read like a reshuffled Recently-added. Give it a discovery identity:

- Weights: drop freshness (0.25 -> 0.12) and bump novelty (0.30 -> 0.35) so
  recency stops dominating and topic-novelty leads.
- Add a soft followed-author penalty (weight 0.3): the scorer now reads
  subscriptionAuthors from the Explore runtime and deprioritizes creators you
  already follow, so non-followed creators surface ("new to you"). A penalty,
  not a filter, so the feed stays full if you follow lots of people.
- More variety: lower the diversity sorter's MMR lambda (0.7 -> 0.5).

Also compacted For You's subscriptionAuthors reader to a one-liner to stay
under feedCoordinator's size cap while adding the same reader to Explore.

Tests: explore follows-penalty scoring; existing explore-scorer/diversity +
feedCoordinator suites green.

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

js/feedEngine/exploreScoring.js

@@ -4,12 +4,13 @@ import { normalizeHashtag } from "../utils/hashtagNormalization.js";
 import { isPlainObject, toSet } from "./utils.js";
 
 const DEFAULT_WEIGHTS = Object.freeze({
-  novelty: 0.3,
-  freshness: 0.25,
+  novelty: 0.35, // reward topics outside your watch history (discovery)
+  freshness: 0.12, // Explore is discovery, not "what's new" — recency matters less
   historySimilarity: 0.2,
   newTagFraction: 0.1,
   popularityNorm: 0.1,
   disinterestOverlap: 0.25,
+  followedPenalty: 0.3, // deprioritize creators you already follow → new-to-you
 });
 
 const DEFAULT_FRESHNESS_HALF_LIFE_DAYS = 14;
@@ -319,6 +320,10 @@ export function createExploreScorerStage({ stageName = "explore-scorer", ...opti
     const popularityMax = resolvePopularityMax(options, context);
     const freshnessHalfLifeDays = resolveFreshnessHalfLife(options, context);
     const now = resolveNow(context);
+    // Soft penalty for creators you already follow so Explore leans toward
+    // creators that are NEW to you. A penalty (not a filter) keeps the feed full
+    // when you follow lots of people.
+    const follows = toSet(context?.runtime?.subscriptionAuthors);
 
     for (const item of items) {
       if (!item || typeof item !== "object") {
@@ -379,13 +384,17 @@ export function createExploreScorerStage({ stageName = "explore-scorer", ...opti
         freshness = clamp01(decay);
       }
 
+      const followedMatch =
+        typeof video.pubkey === "string" && follows.has(video.pubkey) ? 1 : 0;
+
       const score = clamp01(
         weights.novelty * novelty +
           weights.freshness * freshness +
           weights.historySimilarity * historySimilarityPositive +
           weights.newTagFraction * newTagFraction +
           weights.popularityNorm * popularityNorm -
-          weights.disinterestOverlap * disinterestOverlap,
+          weights.disinterestOverlap * disinterestOverlap -
+          (weights.followedPenalty || 0) * followedMatch,
       );
 
       if (!isPlainObject(item.metadata)) {
@@ -400,6 +409,7 @@ export function createExploreScorerStage({ stageName = "explore-scorer", ...opti
         historySimilarity,
         popularityNorm,
         freshness,
+        followedMatch,
         tags: Array.from(videoTags),
       };