Similar versions: surface-overlap metric + endpoint + UI panel

Ranks catalogued engine versions by how much of their CMC_* surface they share,
which (unlike a binary fuzzy hash) stays meaningful across compilers — the golden
pair PIKLIB8/MSVC6 vs bloomoodll/MSVC8 scores 85%.

- similarity.py: jaccard, surface_similarity (per-axis + pooled overall),
  fuzzy_similarity (ssdeep via ppdeep, secondary signal)
- service.similar_snapshots + GET /snapshots/{id}/similar?min=N (SimilarHit)
- UI: "Podobne wersje" panel in the snapshot browser (overlap bar + ⇄ diff)
- tests: 6 new (jaccard, identical/disjoint, golden pair 0<x<100, fuzzy,
  endpoint + min filter) -> 28/28

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

ams/api/service.py

@@ -42,6 +42,30 @@ def _get_or_create_game(db: Session, name: str) -> models.Game:
     return game
 
 
+def similar_snapshots(
+    db: Session, snapshot_id: int, minimum: int = 0
+) -> list[tuple[models.Snapshot, dict]]:
+    """Rank every other catalogued snapshot against #snapshot_id by surface similarity.
+
+    Returns (snapshot, score) pairs (score = ams.similarity report) sorted by `overall` desc,
+    dropping anything below `minimum`. Returns None if the target doesn't exist."""
+    from ..similarity import similarity
+    from ..snapshot import Snapshot as Surface
+
+    target = db.get(models.Snapshot, snapshot_id)
+    if target is None:
+        return None
+    t_surface = Surface(target.data)
+
+    hits: list[tuple[models.Snapshot, dict]] = []
+    for other in db.scalars(select(models.Snapshot).where(models.Snapshot.id != snapshot_id)):
+        score = similarity(t_surface, Surface(other.data))
+        if score["overall"] >= minimum:
+            hits.append((other, score))
+    hits.sort(key=lambda pair: pair[1]["overall"], reverse=True)
+    return hits
+
+
 def import_snapshot(db: Session, data: dict[str, Any], game_name: str | None = None) -> models.Snapshot:
     """Upsert a snapshot, deduped by the binary's sha256 (falling back to a content hash)."""
     sha = data.get("binary", {}).get("sha256") or _content_sha(data)