[b-re-w]

Closes #84

• DSL: PY3_15_0, compileSdk = "3.15" resolves to 3.15.0, trailing t parses as free-threaded (toWheelVersion keeps it; the release string never carries it). 3.14.7 and 3.13.0 kept.
• buildPython forwards compileSdk to pypackpack's BundleRequest.pythonVersion (pypackpack Free-threaded CPython for Android and iOS, built by us (python-multiplatform #163) #77).
• usage-example and CI moved to 3.15.0 / python-multiplatform 3.15.0-alpha01 (python-multiplatform #197).
• Not checked: a 3.15t compileSdk against a GIL-built python-multiplatform (written in SPEC).
• :toolchain:test 282 passed, 0 failed (local, JDK 21, packpack from pypackpack develop in mavenLocal).