🧪 Add negative testing for Engine.greedyNextToken

[void0x14]

🎯 What: The testing gap addressed
I noticed that Engine.greedyNextToken was only tested for positive logits. This left a potential bug where if an array contained only negative or extremely small numbers (like when std.testing.allocator fills memory with 0xaa), the greedy choice could fail.

📊 Coverage: What scenarios are now tested
Added tests explicitly setting negative FP values in an array padded out with -100.0.

✨ Result: The improvement in test coverage
Engine.greedyNextToken is now strictly verified to be robust against negative distributions in the output array.

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PR created automatically by Jules for task 17257759408583250986 started by @ulac000000

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