Add K3Z stand-structure proving-ground surface

[gparadis]

Summary

• add the intensive_light_standstructure K3Z proving-ground surface
• refresh shared BTC/TIPSY managed-curve artifacts with the first stand-structure bank data
• document the proving-ground rollout in the K3Z operator and variant docs

Testing

• sphinx-build -b html docs _build/html -W
• rebuilt tracks_intensive_light_standstructure
• direct smoke of managed stand-structure rows in features.csv and accounts.csv