Three table touches set the table: src/table-plane.ts fits the plane and POST /settings/table-touch applies it #447

Part of #439 (the table plane is wrong; calibrate it by touching the table at 3 points).

Done:

• [ ] Software/so101/src/table-plane.ts is pure. Given three gripper-tip points (metres, base frame), it returns the plane's height at the base and its tilt in degrees. Points that are nearly collinear or less than 5 cm apart are refused with a reason.
• [ ] POST /settings/table-touch {"n": 1|2|3} on src/server.ts stores the gripper tip from forward kinematics of Present_Position (src/kinematics.ts). On n=3 it sets tableOffsetMm from the fit, within -100..100, keeps the margin rule table-here uses, saves, and returns the height and the tilt. A tilt over 3° is reported, not applied.
• [ ] The relay treats it as owner-only.
• [ ] Tests: the fit on synthetic points (flat, tilted, collinear). Three touches against the fake bus set the offset, and a missing touch is a 409.
• [ ] bun --cwd Software/so101 test passes.

Risk: the table is the floor of the safe zone. A wrong fit must be refused rather than applied.

Parked by the crew: the check stayed red through 2 automatic tries (pull #465 closed). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.