The arm server writes settings.servoGains to the servos at start and when a silent servo answers again, only where registers differ #516

From #439: a replaced servo must not be silently back at P16.

Done when:

• [ ] At start, and when a servo that stopped answering answers again, the arm server reads regs 21/22/23 (P/D/I) per joint and writes only the ones that differ from settings.servoGains, so a plain restart costs no EEPROM writes.
• [ ] It never writes while a turn is held, during STOP, E-stop or a hold. It waits and tries again once the arm is free. A gain write moves no joint.
• [ ] The factory values are recorded first through the existing baseline (src/servo-baseline.ts) if none is recorded yet. GET /servo-params shows the live values after the write.
• [ ] A test on the fake bus shows: start → gains written; restart → nothing written; a servo goes silent and comes back at P16 → it is rewritten; a held turn → no write until it ends.
• [ ] bun --cwd Software/so101 test passes.

Out of scope: applying a PUT /settings change live. That waits for the next start or swap, and the README says so in one line.

Files: src/server.ts, src/servo-gains.ts, the bus/servo-params writer, tests. Risk: EEPROM wear, and writing during motion. Both are covered by the tests above. Brief: a builder who knows the servo bus.