Remove legacy RefObject/OutObject wrappers and finalize Java-idiomatic refactor

[Copilot]

This project still carried Tangible wrapper classes used to emulate C# ref/out semantics. This PR completes the migration to Java-idiomatic value/return flow by removing remaining wrapper usage and deleting the obsolete wrapper types.

• Ref/out emulation removal in core paths

  • ClipperBase: replaced wrapper-based swaps and out-params with direct local swaps and typed return values (PopHorz() return, HorzDirection result).
  • ClipperOffset: replaced RefObject<Integer> index threading with int in/out via method return.
  • RectClip64 / RectClipLines64: replaced mutable RefObject call patterns with explicit result carriers (LocationResult, IntersectionResult, NextLocationResult) and state assignment at call sites.

• Wrapper class removal

  • Deleted:
    • src/main/java/tangible/RefObject.java
    • src/main/java/tangible/OutObject.java
  • Updated src/main/java9/module-info.java to remove exports tangible; after package deletion.

• Test alignment

  • TestPolytree: converted recursive counter propagation from wrapper mutation to int return flow.

// before
RefObject<Integer> kRef = new RefObject<>(0);
for (int i = 1; i < highI; i++) {
    OffsetPoint(group, path, i, kRef);
}

// after
int k = 0;
for (int i = 1; i < highI; i++) {
    k = OffsetPoint(group, path, i, k);
}

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