feat: stable macro regime detection — blended signals + Bayesian smoothing

Replace 1-day change_pct with weighted blend (20% 1d / 50% 5d / 30% 10d)
for all 20 scored signals. Add Bayesian prior from rolling 25-day history
(weight 15%→45%) and a 10-point persistence threshold before regime switch.

Bootstrap on first load: replays last 20 trading days via yf.download batch
(45d) to pre-populate _regime_history, so stability is visible immediately.

Frontend: adds 'stable Xj' badge and history depth indicator on regime banner.
Doc: updates v4.0→v4.1, rewrites Étape 1 Régime Macro and glossary entry.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/pages/MacroRegime.tsx

@@ -127,6 +127,8 @@ export default function MacroRegime() {
   const ranked: [string, number][] = scenarios.ranked ?? []
   const reasons: Record<string, string[]> = scenarios.reasons ?? {}
   const assetBias: Record<string, Record<string, string>> = scenarios.asset_bias ?? {}
+  const historyDays: number = scenarios.history_days ?? 0
+  const regimeStability: number = scenarios.regime_stability ?? 0
   const dominantMeta = meta[dominant] ?? { label: dominant, color: '#94a3b8', emoji: '?' }
   const dominantBias: Record<string, string> = assetBias[dominant] ?? {}
 
@@ -215,6 +217,17 @@ export default function MacroRegime() {
                   {scores[dominant]}%
                 </div>
                 <div className="text-xs text-slate-600">score de régime</div>
+                {regimeStability > 0 && (
+                  <div className="text-[10px] mt-1 px-1.5 py-0.5 rounded-full text-center"
+                    style={{ background: `${dominantMeta.color}22`, color: dominantMeta.color }}>
+                    stable {regimeStability}j
+                  </div>
+                )}
+                {historyDays > 0 && (
+                  <div className="text-[10px] text-slate-700 mt-0.5 text-center">
+                    {historyDays}j d'historique
+                  </div>
+                )}
               </div>
             </div>
           )}