Bump python-multiplatform and typedpython-gate to 3.15.0-alpha02 #119

What & Why

python-multiplatform v3.15.0-alpha02 has been released (adding Windows MSVC support for TypedPython, dropping the # typedpython: compiled module pragma in favour of @compiled only, and clarifying that typedpython is shipped via the GitHub Release and distribution rather than published to PyPI).

Update toolchain to match:

• Bump DEFAULT_TYPEDPYTHON_GATE (TypedpythonGate.kt), usage-example/gradle/libs.versions.toml, AGENTS.md, docs/SPEC.md, .github/workflows/test.yml, TypedpythonGateTest.kt, and TypedpythonGateArtifactTest.kt from 3.15.0-alpha01 to 3.15.0-alpha02.
• Update the comment in toolchain/build.gradle.kts from "which is not on PyPI yet" to "which is not published to PyPI".

Completion Criteria

• :toolchain:test --tests *TypedpythonGateTest --tests *TypedpythonGateArtifactTest --tests *TypedpythonDecisionsTest --tests *TypedpythonNoWheelTest passes against 3.15.0-alpha02.
• ./gradlew :toolchain:publishToMavenLocal and ./gradlew -p usage-example typedpythonCheck pass.