⚡ Optimize ComputeDerivativesFunction to reduce allocation churn

[ysdede]

💡 What:

• Implemented per-thread caching for cache_bitmap_Ix and cache_bitmap_Iy arrays in ComputeDerivativesFunction within JavaImageFunctions.java.
• Updated init() to initialize the cache storage.
• Updated apply() to lazily allocate/resize these arrays as needed and reuse them across calls.

🎯 Why:

• The previous implementation allocated two large int arrays (size width * chunk_height) for every chunk processed by every thread.
• For a typical image (e.g., 2000px wide) processed in 64px chunks, this resulted in dozens of megabytes of allocation churn per frame/image processed.
• Caching these arrays significantly reduces Garbage Collection (GC) pressure and allocation overhead, improving performance, especially on lower-end devices.

📊 Measured Improvement:

• Verified that the changes compile successfully.
• While direct benchmarking on the host was not possible due to environment limitations (missing Android runtime), the optimization is algorithmic: O(1) allocation vs O(N) allocation where N is the number of chunks.
• The logic ensures thread safety by using thread-indexed arrays, consistent with the existing parallelism model in JavaImageProcessing.

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PR created automatically by Jules for task 3011847838455692420 started by @ysdede

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch perf-optimization-derivatives-cache-3011847838455692420

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