settings.servoGains lands from the reviewed branch: validated at PUT /settings, proved by the branch's own server, not the preview #544

Replaces #515. Pull #536 (branch smith/515-settings-servogains-holds-the-measured-p, e53e109) had its code approved in three reviews. It went round only because the reviewer checked the preview's GET /settings, and that answer comes from the Pi's arm server running main (ARM_SERVER). It cannot show a branch's settings.

Done when:

• [ ] Start from that branch and merge main: src/servo-gains.ts, Settings.servoGains, the strict PUT /settings validation (integers 0–254, joints 1–5, the gripper and unknown keys refused), and the README row.
• [ ] src/server.test.ts starts the branch's own arm server with an empty data dir. Its GET /settings equals DEFAULT_SERVO_GAINS. A partial PUT survives a restart. A settings.json without servoGains loads the defaults.
• [ ] bun --cwd Software/so101 test passes, and the description pastes the output of bun --cwd Software/so101 test src/servo-gains.test.ts src/server.test.ts.
• [ ] The description says in one line that the preview's /settings is the Pi's and will not show servoGains until the Pi is deployed. The review is judged on the tests.
• [ ] Nothing is written to a servo.

Brief: any builder, one small pull.