feat: /bridge hub page

[Flotapponnier]

Summary

• Adds /bridge hub landing page combining the two live bridge benches
• src/lib/bridge-hub-stats.ts: fetches both bridge-fee and bridge-quote-latency blobs, joins by provider slug into a unified BridgeHubData shape
• src/app/bridge/page.tsx: server component with 4 summary cards, cost ranking table (sorted by fee p50 asc), quote speed ranking table (sorted by latency p50 asc), architecture explainer (intent / aggregator / protocol), corridor list, methodology footer
• src/lib/sitemap-builder.ts: adds /bridge at priority 0.9

No Prometheus queries at render time, no new blob format, no materializer changes needed. Two existing bench blobs are already written by the worker.

Test plan

• Check /bridge renders on staging with summary cards showing live data
• Confirm cost table sorted by fee p50 asc (cheapest first)
• Confirm quote speed table sorted by latency p50 asc
• Verify graceful empty state renders if both blobs missing
• Sitemap includes /bridge entry