[b-re-w]

Part of #191 (Phase A).

Summary

• Adds Kotlin/Native desktop targets macosArm64 and linuxX64 to :python-multiplatform under nativeMain -> desktopNativeMain -> {macosArm64Main, linuxX64Main} and nativeTest -> desktopNativeTest -> {macosArm64Test, linuxX64Test} (mingwX64 excluded; commonMain, iosMain, and artMain untouched).
• Configures cinterop and test binary linker flags using libVersion and abiSuffix (-lpython$libVersion$abiSuffix) against the acquired python-build-standalone trees, staging the resolved Linux shared library via stageDesktopLibraries.
• Implements DesktopNativePythonHome (applyPackagedPythonHome) and discoverStagedPayloadRoots in desktopNativeMain, and platform/thread/executable-directory actuals in macosArm64Main and linuxX64Main.
• Adds desktop-native-linux job in .github/workflows/desktop.yml to run :python-multiplatform:linuxX64Test on ubuntu-latest.

Verification

• :python-multiplatform:macosArm64Test: 496 tests, 496 passed, 0 failed, 0 skipped (66 suites)
• :python-multiplatform:compileKotlinAndroidNativeArm64: passed
• :python-multiplatform:compileKotlinIosSimulatorArm64: passed
• :python-multiplatform:linkDebugTestLinuxX64: passed