Add VAM (climbing velocity) metric and per-duration curve

Extract pipeline now computes two VAM metrics per activity (cycling,
running, hiking, walking):
- climbing_vam_mh: VAM on ascending segments only, using 30 s forward
  lookahead to classify climbing vs. flat/descent (stored in detail JSON)
- vam_curve: [[duration_s, vam_mh], ...] best VAM per standard duration
  (60 s – 1 h), sliding window on 30 s smoothed elevation, only windows
  with ≥ 10 m net gain count (stored in summary + detail)

Athlete JSON aggregates vam_curve across all activities (all_time,
last_365d, last_90d), same structure as power_curve.

Frontend:
- ActivityDetail shows "Climbing VAM" stat (grouped with elevation)
- AthleteView adds a "VAM Curve" tab that appears only when the athlete
  has climbing data; renders VamChart (new component, mirrors MmpChart)

vam_curve stripped from combined global feed; kept in user year shards
for season-based on-the-fly aggregation in VamChart.

Requires bincio reextract to backfill existing activities.

bincio/render/cli.py

@@ -116,7 +116,7 @@ def _rebuild_athlete_json(data: Path, handle: str | None = None) -> None:
     from bincio.render.merge import parse_sidecar, _apply_sidecar_summary
 
     targets = [data / handle] if handle else _user_dirs(data)
-    _COMPUTED = {"bas_version", "generated_at", "power_curve", "records", "best_climbs"}
+    _COMPUTED = {"bas_version", "generated_at", "power_curve", "vam_curve", "records", "best_climbs"}
     for user_dir in targets:
         index_path = user_dir / "index.json"
         if not index_path.exists():