VAM: add --recompute-vam flag and compute_vam_from_timeseries helper

Refactors core VAM logic into _vam_from_ele_1hz() and _build_ele_1hz()
so both the DataPoint-based extract path and the timeseries-based backfill
path share the same implementation.

render --recompute-vam reads stored *.timeseries.json files and updates
climbing_vam_mh + vam_curve in activities/*.json and index.json in-place,
without re-parsing the original FIT/GPX files.

bincio/render/cli.py

@@ -377,6 +377,58 @@ def _link_data(site: Path, data: Path) -> None:
     console.print(f"Linked data: [cyan]{target}[/cyan] → [cyan]{public_data}[/cyan]")
 
 
+def _recompute_vam(data: Path, handle: str | None = None) -> None:
+    """Recompute climbing_vam_mh and vam_curve for all activities from stored timeseries."""
+    import json
+    from bincio.extract.metrics import compute_vam_from_timeseries
+
+    targets = [data / handle] if handle else _user_dirs(data)
+    for user_dir in targets:
+        acts_dir = user_dir / "activities"
+        index_path = user_dir / "index.json"
+        if not acts_dir.exists() or not index_path.exists():
+            continue
+        try:
+            index_data = json.loads(index_path.read_text(encoding="utf-8"))
+        except Exception:
+            continue
+
+        updated = 0
+        for act_path in acts_dir.glob("*.json"):
+            if act_path.stem.endswith((".timeseries", ".geojson")):
+                continue
+            ts_path = acts_dir / f"{act_path.stem}.timeseries.json"
+            if not ts_path.exists():
+                continue
+            try:
+                detail = json.loads(act_path.read_text(encoding="utf-8"))
+                sport = detail.get("sport", "other")
+                ts = json.loads(ts_path.read_text(encoding="utf-8"))
+                new_vam, new_curve = compute_vam_from_timeseries(ts, sport)
+                if (new_vam == detail.get("climbing_vam_mh")
+                        and new_curve == detail.get("vam_curve")):
+                    continue
+                detail["climbing_vam_mh"] = new_vam
+                detail["vam_curve"] = new_curve
+                act_path.write_text(
+                    json.dumps(detail, indent=2, ensure_ascii=False), encoding="utf-8"
+                )
+                act_id = act_path.stem
+                for s in index_data.get("activities", []):
+                    if s.get("id") == act_id:
+                        s["vam_curve"] = new_curve
+                        break
+                updated += 1
+            except Exception:
+                pass
+
+        if updated:
+            index_path.write_text(
+                json.dumps(index_data, indent=2, ensure_ascii=False), encoding="utf-8"
+            )
+        console.print(f"  [cyan]{user_dir.name}[/cyan]: {updated} activity(ies) updated")
+
+
 @click.command()
 @click.option("--config", "config_path", default=None,
               help="Path to extract_config.yaml (reads output.dir from it).")
@@ -400,6 +452,9 @@ def _link_data(site: Path, data: Path) -> None:
 @click.option("--recompute-elevation", "recompute_elevation", is_flag=True,
               help="Recompute elevation_gain_m/loss_m for all activities from stored timeseries "
                    "(run once after upgrading the dropout-skip fix).")
+@click.option("--recompute-vam", "recompute_vam", is_flag=True,
+              help="Recompute climbing_vam_mh and vam_curve for all activities from stored "
+                   "timeseries (run once after adding VAM support).")
 def render(
     config_path: Optional[str],
     data_dir: Optional[str],
@@ -411,6 +466,7 @@ def render(
     no_build: bool,
     recompute_climbs: bool,
     recompute_elevation: bool,
+    recompute_vam: bool,
 ) -> None:
     """Build (or serve) the BincioActivity static site from a BAS data store."""
 
@@ -428,6 +484,10 @@ def render(
         console.print("Recomputing elevation gain/loss from timeseries…")
         _recompute_elevation(data, handle=handle)
 
+    if recompute_vam:
+        console.print("Recomputing VAM from timeseries…")
+        _recompute_vam(data, handle=handle)
+
     _merge_edits(data, handle=handle)
     _rebuild_athlete_json(data, handle=handle)
     _bake_tracks(data, handle=handle)