/precision reports the relay→Pi and Pi read-back hops on a clock the relay aligns with the arm server #481

Piece 2 of 3 of #458.

Done:

• [ ] The relay runs the four-timestamp ping from src/hops.ts against the arm server every few seconds over its existing link and keeps the offset.
• [ ] The relay stamps each forwarded command with its send time. The arm server records relay→Pi from that stamp, on the aligned clock. Unstamped frames, such as the CLI leader's, are still accepted and not counted.
• [ ] Each status carries the arm server's read time, so read-back→relay is measured too.
• [ ] Both hops appear with p50/p95 in hops of GET /precision. latencyP95Ms adds them, and bench/precision.ts prints them (loopback there).
• [ ] Tests in relay.test.ts and the precision tests use a fake arm server with a fixed clock skew.
• [ ] Merge main first. This piece touches precision.ts, server.ts and relay.ts, and nothing on the page. bun --cwd Software/so101 test passes.