⚡ Bolt: Optimize TripletDataGenerator sampling

[google-labs-jules]

💡 What: Replaced O(N) list comprehensions in the batch generation loop with O(1) dictionary lookups and rejection sampling.
🎯 Why: The previous implementation scanned the entire dataset twice for every sample in a batch, causing massive slowdowns as dataset size increased.
📊 Impact: Reduced batch generation time by ~88% (from 1.27s to 0.15s per batch for 20k images).
🔬 Measurement: Verified with a benchmark script using mocked image loading.

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PR created automatically by Jules for task 363534553400923119 started by @Devasy23

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