www: default availSrc → v3 (avail-v3 min-cover pyramid)

Station avail charts default to `/api/avail-v3` (inventory-driven
planner, pyrmts `ffc72db`): ~1.3 s chart XHR in-browser vs ~3-5 s on
the legacy `/api/totals` path, no more phantom-key 500s at rung
boundaries. `?availSrc=totals` (or the flags panel) remains as the
escape hatch until #108 retires the legacy availability reader.

Re-flips the #111 default that was reverted during the /1m partials
rollback (`287bed61` era) — the read path is now correct under
min-cover maintenance.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: ryan-williams

www/src/contexts/FlagsContext.tsx

@@ -5,7 +5,11 @@ import { createContext, ReactNode, useCallback, useContext, useMemo, useState }
 // value type from `options`.
 export const FLAGS = {
   availSrc: {
-    default: 'totals' as const,
+    // 'v3' = avail-v3 min-cover pyramid via the inventory-driven planner
+    // (pyrmts `ffc72db`); ~1.3s chart XHR vs ~3-5s on the legacy
+    // 'totals' path. 'totals' remains as the escape hatch
+    // (`?availSrc=totals`) until #108 retires the legacy reader.
+    default: 'v3' as const,
     options: ['totals', 'v3'] as const,
     description: 'Per-station availability backend',
   },