feat(quality): YouTube relevance scoring and cross-source linking

YouTube videos now get real relevance scores based on token overlap
between the search query and video title (was hardcoded at 0.7).
Uses ratio overlap with stopword removal, floored at 0.1.

Cross-source linking annotates items that discuss the same story
across different platforms (e.g., Reddit + HN + X). Items get
bidirectional cross_refs displayed as [xref: R3, HN5] in compact
output so Claude can triangulate multi-platform coverage.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

scripts/lib/dedupe.py

@@ -36,7 +36,11 @@ def jaccard_similarity(set1: Set[str], set2: Set[str]) -> float:
     return intersection / union if union > 0 else 0.0
 
 
-def get_item_text(item: Union[schema.RedditItem, schema.XItem, schema.YouTubeItem, schema.HackerNewsItem]) -> str:
+AnyItem = Union[schema.RedditItem, schema.XItem, schema.YouTubeItem,
+                schema.HackerNewsItem, schema.WebSearchItem]
+
+
+def get_item_text(item: AnyItem) -> str:
     """Get comparable text from an item."""
     if isinstance(item, schema.RedditItem):
         return item.title
@@ -44,10 +48,23 @@ def get_item_text(item: Union[schema.RedditItem, schema.XItem, schema.YouTubeIte
         return item.title
     elif isinstance(item, schema.YouTubeItem):
         return f"{item.title} {item.channel_name}"
+    elif isinstance(item, schema.WebSearchItem):
+        return item.title
     else:
         return item.text
 
 
+def _get_cross_source_text(item: AnyItem) -> str:
+    """Get text for cross-source comparison.
+
+    Same as get_item_text() but truncates X posts to 100 chars
+    to level the playing field against short Reddit/HN titles.
+    """
+    if isinstance(item, schema.XItem):
+        return item.text[:100]
+    return get_item_text(item)
+
+
 def find_duplicates(
     items: List[Union[schema.RedditItem, schema.XItem]],
     threshold: float = 0.7,
@@ -138,3 +155,42 @@ def dedupe_hackernews(
 ) -> List[schema.HackerNewsItem]:
     """Dedupe Hacker News items."""
     return dedupe_items(items, threshold)
+
+
+def cross_source_link(
+    *source_lists: List[AnyItem],
+    threshold: float = 0.5,
+) -> None:
+    """Annotate items with cross-source references.
+
+    Compares items across different source types using Jaccard similarity
+    on char trigrams. When similarity exceeds threshold, adds bidirectional
+    cross_refs with the related item's ID. Modifies items in-place.
+
+    Args:
+        *source_lists: Variable number of per-source item lists
+        threshold: Similarity threshold for cross-linking (default 0.5)
+    """
+    all_items = []
+    for source_list in source_lists:
+        all_items.extend(source_list)
+
+    if len(all_items) <= 1:
+        return
+
+    # Pre-compute trigrams using cross-source text extraction
+    ngrams = [get_ngrams(_get_cross_source_text(item)) for item in all_items]
+
+    for i in range(len(all_items)):
+        for j in range(i + 1, len(all_items)):
+            # Skip same-source comparisons (handled by per-source dedupe)
+            if type(all_items[i]) is type(all_items[j]):
+                continue
+
+            similarity = jaccard_similarity(ngrams[i], ngrams[j])
+            if similarity >= threshold:
+                # Bidirectional cross-reference
+                if all_items[j].id not in all_items[i].cross_refs:
+                    all_items[i].cross_refs.append(all_items[j].id)
+                if all_items[i].id not in all_items[j].cross_refs:
+                    all_items[j].cross_refs.append(all_items[i].id)