Steltic v0.1.3

Engine deps (openseespy/numpy/scipy/matplotlib) are now core dependencies - designs run out of the box without Docker or extras. Boot-time check warns with the exact fix if engine packages are missing.

Upgrading from 0.1.1/0.1.2: run uv tool uninstall steltic then uv tool install steltic - a plain upgrade keeps the old Python interpreter; a fresh install picks a compatible 3.12.