Recognize the Darwin platform alias as macOS

[toaster0123]

Problem

The Windows check uses contains("win"), which also matches darwin. Both the runtime loader and core Gradle native-project selection therefore take the Windows branch for the Darwin alias, making the subsequent explicit Darwin/macOS branch unreachable.

Change

Use startsWith("win") in both selectors and add a regression covering Linux, Windows 11, Mac OS X, and Darwin. The regression restores all changed system properties.

Scope and severity

This is a compatibility-alias correction. The usual Mac OS X value already worked; this review did not demonstrate a supported macOS JVM reporting Darwin. It should not be described as a general macOS loading failure.

Validation

• Original selector reproduced Darwin -> windows
• New JUnit platform-detection regression passes all four labels
• Benign locally compiled fixture library passes bundled resource/JAR loading, explicit file/directory overrides, java.library.path fallback, and normal-exit extraction cleanup on Linux
• Java source/target 8 compilation and git diff --check passed

Limits

The Gradle selector was source-reviewed, not executed with Gradle. Full Gradle/CMake and platform CI were not run locally. No macOS or Windows runtime was available; mocked system properties exercise classification only.