perf: add patch_index flag to recalculate_elevation_hysteresis

Allows bulk callers to skip per-activity index.json rewrites and
batch the update themselves, reducing O(n²) index churn to O(n).

bincio/extract/dem.py

@@ -299,7 +299,9 @@ def recalculate_elevation(
     }
 
 
-def recalculate_elevation_hysteresis(user_dir: Path, activity_id: str) -> dict:
+def recalculate_elevation_hysteresis(
+    user_dir: Path, activity_id: str, *, patch_index: bool = True
+) -> dict:
     """Recompute elevation gain/loss from the original recorded elevation data.
 
     Algorithm
@@ -370,7 +372,8 @@ def recalculate_elevation_hysteresis(user_dir: Path, activity_id: str) -> dict:
     detail["elevation_loss_m"] = loss_r
     json_path.write_text(json.dumps(detail, indent=2, ensure_ascii=False), encoding="utf-8")
 
-    # Patch index.json summary
+    # Patch index.json summary (skip for bulk callers who batch this themselves)
+    if patch_index:
         index_path = user_dir / "index.json"
         if index_path.exists():
             index = json.loads(index_path.read_text(encoding="utf-8"))