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.

site/src/components/AthleteView.svelte

@@ -2,6 +2,7 @@
   import { onMount } from 'svelte';
   import type { AthleteJson, BASIndex, ActivitySummary } from '../lib/types';
   import MmpChart from './MmpChart.svelte';
+  import VamChart from './VamChart.svelte';
   import RecordsView from './RecordsView.svelte';
   import AthleteDrawer from './AthleteDrawer.svelte';
   import Explore from './Explore.svelte';
@@ -19,12 +20,13 @@
 
   let athlete: AthleteJson | null = null;
   let activities:    ActivitySummary[] = [];
+  let vamActivities: ActivitySummary[] = [];
   let allActivities: ActivitySummary[] = [];
   let loading = true;
   let error: string | null = null;
   let drawerOpen = false;
 
-  type Tab = 'power' | 'records' | 'segments' | 'profile' | 'explore' | 'nerd';
+  type Tab = 'power' | 'vam' | 'records' | 'segments' | 'profile' | 'explore' | 'nerd';
   let activeTab: Tab = 'power';
   let mounted = false;
   let isOwner = false;
@@ -94,7 +96,7 @@
         isOwner = (e as CustomEvent<string>).detail === handle;
       }, { once: true });
     }
-    const TABS: Tab[] = ['power', 'records', 'segments', 'profile', 'explore', 'nerd'];
+    const TABS: Tab[] = ['power', 'vam', 'records', 'segments', 'profile', 'explore', 'nerd'];
     const rawTab = new URLSearchParams(window.location.search).get('tab');
     const resolved = TABS.includes(rawTab as Tab) ? (rawTab as Tab) : 'power';
     activeTab = (resolved === 'explore' && !isOwner) ? 'power' : resolved;
@@ -131,6 +133,7 @@
       athlete = resolvedAthlete;
       allActivities = index.activities.filter(a => !isUnlisted(a.privacy));
       activities    = allActivities.filter(a => a.mmp);
+      vamActivities = allActivities.filter(a => a.vam_curve);
     } catch (e: any) {
       error = e.message;
     } finally {
@@ -156,15 +159,19 @@
     return hi >= 900 ? `${lo}+ bpm` : `${lo}–${hi} bpm`;
   }
 
-  const ALL_TABS: { key: Tab; label: string; ownerOnly?: boolean }[] = [
+  const ALL_TABS: { key: Tab; label: string; ownerOnly?: boolean; requiresVam?: boolean }[] = [
     { key: 'power',    label: 'Power Curve' },
+    { key: 'vam',      label: 'VAM Curve',    requiresVam: true },
     { key: 'records',  label: 'Records'     },
     { key: 'segments', label: 'Segments'    },
     { key: 'profile',  label: 'Profile'     },
     { key: 'explore',  label: 'Explore',      ownerOnly: true },
     { key: 'nerd',     label: 'Nerd Corner',  ownerOnly: true },
   ];
-  $: TABS = ALL_TABS.filter(t => !t.ownerOnly || isOwner);
+  $: TABS = ALL_TABS.filter(t =>
+    (!t.ownerOnly || isOwner) &&
+    (!t.requiresVam || athlete?.vam_curve?.all_time != null)
+  );
 </script>
 
 {#if loading}
@@ -216,6 +223,16 @@
       <p class="text-zinc-500 text-sm">No power data found. Make sure your activities include power meter data.</p>
     {/if}
 
+  <!-- VAM Curve tab -->
+  {:else if activeTab === 'vam'}
+    {#if athlete.vam_curve?.all_time}
+      <div class="bg-zinc-900 rounded-xl p-4 border border-zinc-800">
+        <VamChart {athlete} activities={vamActivities} />
+      </div>
+    {:else}
+      <p class="text-zinc-500 text-sm">No climbing data found.</p>
+    {/if}
+
   <!-- Records tab -->
   {:else if activeTab === 'records'}
     <RecordsView {athlete} {base} />