feat(sprint-1-5): cierre Etapa 2 - formatPayback + fixes de marca

- Helper formatPayback con 12 casos cubiertos
- Aplicado en RoiKpiBar, drawRoiKpiCards, csv-export
- Auditoría WCAG: ratio 2.22:1, opción D (font-bold) para Etapa 3
- Adelanto: strings 'InQ ROI' y 'Powered by InQuality' en footers PDF
- Threshold ROI: '1.000%' sin decimal
- 455 tests verdes + 2 E2E

Refs: sprints/sprint-1-5/ETAPA_2_PROMPT.md

src/features/report/HeatmapLegend.tsx

@@ -1,5 +1,5 @@
 import { formatCurrency, formatPercent } from '@/lib/format'
-import { heatmapLegendBounds } from '@/lib/colors'
+import { heatmapLegendBounds, HEATMAP_LOW_HEX, HEATMAP_MID_HEX, HEATMAP_HIGH_HEX } from '@/lib/colors'
 import type { HeatmapMode } from '@/lib/colors'
 import type { ActivitySimResult } from '@/domain/types'
 
@@ -21,7 +21,7 @@ export function HeatmapLegend({ perActivity, mode, currency }: HeatmapLegendProp
     <div className="flex items-center gap-3 mt-3">
       <span className="text-xs text-slate-500 shrink-0">Bajo costo</span>
       <div className="flex-1 h-3 rounded-full" style={{
-        background: 'linear-gradient(to right, #10b981, #f59e0b, #ef4444)'
+        background: `linear-gradient(to right, ${HEATMAP_LOW_HEX}, ${HEATMAP_MID_HEX}, ${HEATMAP_HIGH_HEX})`
       }} />
       <span className="text-xs text-slate-500 shrink-0">Alto costo</span>
       <div className="h-4 border-l border-slate-200 mx-1" />