cornerstones: tracking — tuning fork / bimetallic thermostat / coupon ladder

[ecto]

Physical artifacts with brutal external oracles, each forcing tower arrows:

• Tuning fork (RT0–RT4): DONE through order placement — see PR feat(examples): tuning-fork family — reality-tower cornerstone 1 #478; measurement pending → first RT7 seed data.
• Bimetallic thermostat (RT3/RT6): curvature vs temperature from two MaterialCards (CTE = pure fluctuation quantity), geometry optimized by the cross-scale gradient. Oracle: heat gun + dial indicator.
• Calibration coupon ladder (RT7): standard coupon set; every measurement tightens posteriors; prediction-interval coverage as the oracle.

Blocked by (thermostat) #505; (ladder) #506.