feat: selection mode filtering with av fallback

hef/selection.py

@@ -35,3 +35,21 @@ def distance(a: Coordinate, b: Coordinate, weights: Weights = Weights()) -> floa
 def record_coordinate(record: Record) -> Coordinate:
     """The (left, right, dark, light) coordinate of a catalog record."""
     return Coordinate(record.left, record.right, record.dark, record.light)
+
+
+def candidates_for_mode(records, mode: str, pool_size: int) -> list[Record]:
+    """Records eligible for a content mode.
+
+    For 'av', if fewer than pool_size native 'av' records exist, fall back to
+    including 'audio' and 'video' records so the pool is never starved.
+    """
+    if mode not in CONTENT_MODES:
+        raise ValueError(
+            f"candidates_for_mode expects a content mode {sorted(CONTENT_MODES)}, "
+            f"got {mode!r}"
+        )
+    primary = [r for r in records if r.mode == mode]
+    if mode == "av" and len(primary) < pool_size:
+        extra = [r for r in records if r.mode in {"audio", "video"}]
+        return primary + extra
+    return primary