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
@@ -370,17 +372,18 @@ 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
-    index_path = user_dir / "index.json"
-    if index_path.exists():
-        index = json.loads(index_path.read_text(encoding="utf-8"))
-        for s in index.get("activities", []):
-            if s.get("id") == activity_id:
-                s["elevation_gain_m"] = gain_r
-                break
-        index_path.write_text(
-            json.dumps(index, indent=2, ensure_ascii=False), encoding="utf-8"
-        )
+    # 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"))
+            for s in index.get("activities", []):
+                if s.get("id") == activity_id:
+                    s["elevation_gain_m"] = gain_r
+                    break
+            index_path.write_text(
+                json.dumps(index, indent=2, ensure_ascii=False), encoding="utf-8"
+            )
 
     return {
         "elevation_gain_m": gain_r,