Add JVM lazy clipboard verification tests for platform behavior documentation

[guiyanakuang]

JVM's Transferable.getTransferData() is not lazy on macOS and Windows — setContents() immediately calls getTransferData() to serialize data into the system clipboard. Only Linux/X11 supports true lazy evaluation.

Add tests that prove this platform behavior using a custom Transferable that tracks getTransferData() invocations. Tests are organized by platform using JUnit 5 @Nested + @EnabledOnOs, and safely skip in headless CI environments via @EnabledIf.

• macOS: 4 tests asserting eager getTransferData during setContents
• Windows: 4 tests asserting eager behavior (mirrors macOS)
• Linux: 4 tests asserting lazy behavior (deferred until getData)