Publish the embedded CPython version so build tools can check against it #61

toolchain (toolchain#42) stops fetching an interpreter and instead checks its compileSdk against the CPython python-multiplatform embeds. It needs to read that version.

Decided: both
(a) same build / includeBuild: the :python-multiplatform project exposes it as a Gradle extension property (e.g. pythonMultiplatform.pythonVersion, plus the free-threaded flag), next to cpythonIncludeDirectories;
(b) published consumers: the artifact carries it — a Gradle module metadata attribute and a META-INF/python-multiplatform/python.properties resource (pythonVersion, freeThreaded, and per-target actual PY_VERSION once #47 makes them equal) — so a consumer can read it from the resolved artifact.

Completion: tests for both; toolchain can read (a) in a composite build and (b) from a mavenLocal-published artifact.