⚡ perf: extract inline array allocation in Heatmap component

[artosien]

💡 What: Extracted the inline array [0, 0, 0, 0, 0, 0] into a const EMPTY_SET outside the Heatmap component to prevent re-allocating a new array on every render.
🎯 Why: To reduce unnecessary memory allocations and garbage collection pressure, improving render performance for the Heatmap component.
📊 Measured Improvement: A micro-benchmark (benchmark.js) over 10M iterations showed a ~10x performance improvement (~167ms vs ~16ms) in allocating vs caching an array of this size, reducing overhead.

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PR created automatically by Jules for task 17576245997045101317 started by @artosien

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