The relay holds one LiveMetrics, reports the Pi round trip from arm-link, and serves it at GET /metrics.prom #245

Split from #110 (second half of #88), piece 1 of 3. On main, src/live-metrics.ts exists but nothing feeds it. Both of #110's parks were the preview environment failing within seconds of the pull opening, not the change. Pull #244's branch holds a complete attempt that can be reused in part.

Done:

• [ ] Software/so101/src/relay.ts holds one LiveMetrics, and src/arm-link.ts reports each round trip to it (observe("pi_rtt_ms", …)).
• [ ] GET /metrics.prom answers text/plain; version=0.0.4 with the security headers, and the existing JSON GET /metrics is unchanged.
• [ ] A relay test with a fake arm that waits 50 ms shows the delay in the round-trip p95 on /metrics.prom.
• [ ] bun x tsc --noEmit and bun --cwd Software/so101 test pass. forge.json is untouched.