Expose each target's CPython link library directory (libDir) next to includeDir #56

Follow-up to #46, requested by pypackpack (compile slot) and TypedPython: Android must link libpython explicitly (-lpython3.14; Bionic does not resolve undefined symbols from the host process), Windows needs python314.lib. macOS/Linux build with -undefined dynamic_lookup and need no lib dir. November scope needs only android-aarch64 (and android-x86_64 for the emulator); Windows is listed but not required now.

Add libDir(target, flavour): Provider<Directory> (and the library file name) to CPythonIncludeDirectories, same dependency-carrying rules as includeDir; a target with no link library answers isLinkRequired=false instead of a path.

Completion: unit tests over the observed acquisition tree (android prefix/lib contains libpython3.14.so; windows libs/python314.lib), contract added to docs/platforms/python-version-acquisition.md §7.