Provisional: extensible random generators and splitting for Gecode 7 (breaking changes)

[zayenz]

Provisional design for Gecode 7 or another release permitting breaking changes. Not ready to merge or suitable for Gecode 6.x: APIs, binary compatibility, seeded sequences, and randomized choice archives change.

Add extensible random generators with complete-state replay and reproducible indexed splitting. SplitMix is the default; xorshift64* is an alternative. Generators are stored by value, and randomized choices record the selector states needed for recomputation. Cloning does not split streams; model and custom-brancher transitions remain explicit.

Local Apple Clang 21 ARM64 measurements put SplitMix near the old cloning time, with about 16% overhead in the controlled recomputation workload. Xorshift64* was slower. Engine selection and performance tradeoffs remain under review. Recorded-state replay does not guarantee parallel solution order.

Contracts and migration guidance are in Doxygen and the changelog; the custom-engine example and benchmark tool provide usage instructions.

[zayenz]

• RandomOption now stores validated state as Rnd and returns a copy, avoiding serialization and parsing in the accessor.
• Documented selector hook buffer sizes, pointer advancement, capture timing, and restore/split behavior.
• Completed override annotations in the two affected selector classes. This removes the missing-override warnings.