Curation v2 step 3: Voyage embeddings, cheap signals, candidate telemetry

- embedding.rs: EmbeddingBackend + VoyageBackend, batched bounded-concurrency
  client with its own UsageMeter, f32 BLOB codec, article/interest embedding
  cache keyed by model, dimension and sha256 of the embedded text.
- signals.rs: z-scored interest match, decayed rated-neighbour preference
  with the knn gate, feed affinity with the feed gate, social, text heuristic
  without social terms, mid-rank percentile normalizer, preliminary blend.
- telemetry.rs: candidate_runs writer with §7.5 signals_json, explain and
  near-misses renderers, prune.
- [voyage] and the full [curation.ranking] config with validation.
- CLI: explain, features backfill|prune, generate --skip-embeddings.
- Pipeline: hygiene rows, embed and signals stages before the old prefilter;
  same-date regeneration no longer excludes its own picks.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01A1rCLQeKBgnBo3oTgHuTMe

src/curate/prefilter.rs

@@ -90,7 +90,7 @@ impl PrefilterContext {
         let since = today
             .checked_sub(jiff::Span::new().days(STALE_LOOKBACK_DAYS))
             .unwrap_or(today);
-        let already_published = db.previously_published_ids().await?;
+        let already_published = db.previously_published_ids_before(today).await?;
         let recently_rejected = db.recently_low_scored_ids(STALE_LOW_SCORE, since).await?;
         tracing::debug!(
             published = already_published.len(),
@@ -197,6 +197,29 @@ pub fn social_points(social_score: f64) -> f64 {
     MAX_SOCIAL_POINTS * (social_score / SOCIAL_SATURATION).min(1.0).sqrt()
 }
 
+/// Text-only heuristic used by personalized ranking (§9.3).
+pub fn text_heuristic(article: &Article) -> f64 {
+    longform_points(article.word_count)
+        - excerpt_only_penalty(article)
+        - roundup_penalty(&article.title)
+}
+
+pub fn excerpt_only_penalty(article: &Article) -> f64 {
+    if article.excerpt_only {
+        EXCERPT_ONLY_PENALTY
+    } else {
+        0.0
+    }
+}
+
+pub fn roundup_penalty(title: &str) -> f64 {
+    if looks_like_roundup(title) {
+        ROUNDUP_TITLE_PENALTY
+    } else {
+        0.0
+    }
+}
+
 /// Score one article 0–100 from word count, social proof, source signals,
 /// and the excerpt/roundup/blocklist penalties (§3.5).
 pub fn score_article(article: &Article, _ctx: &PrefilterContext, cfg: &Config) -> f64 {