Add composable Scene2D DSL factory prototype

[marcphilippebeaujean-abertay]

Summary

• add mount with separated placement and actor initialization
• add reusable Scene2dFactoryDescriptor recipes and member-extension invocation
• migrate table to separated callbacks
• add focused tests and documentation

Verification

• JAVA_TOOL_OPTIONS="-Dnet.bytebuddy.experimental=true" ./gradlew :scene2d:check --rerun-tasks
• focused Scene2dFactoryTest passes
• full repository ./gradlew test --rerun-tasks reached unchanged app:ProfilingTest and failed its timing assertions (0.01207 vs 0.01); no app files changed

Java 23 emits non-fatal JaCoCo class-version warnings.

[marcphilippebeaujean-abertay]

Ooops, I was working on this concept on my local computer and the agent decided to open a pr - my bad.