TTSKit: sampleFromProbs crashes when top-k probabilities sum to zero #450

Description

GreedyTokenSampler.sampleFromProbs in Sources/TTSKit/Utilities/Sampling.swift crashes with:

Fatal error: Can't get random value with an empty range

when all top-k probabilities collapse to zero after softmax, making probSum == 0 and Float.random(in: 0..<0) an empty range.

Steps to reproduce

Run TTS generation on long-form text (e.g. audiobook chapters) with temp=0.10, topK=15. After hundreds of chunks, softmax occasionally produces a distribution where all top-k values round to Float zero.

Location

https://github.com/argmaxinc/WhisperKit/blob/d447d30/Sources/TTSKit/Utilities/Sampling.swift#L238-L243

let probSum = probsArray.reduce(0, +)
let randomValue = Float.random(in: 0..<probSum, using: &rng)  // 💥 when probSum == 0

Suggested fix

Guard against a zero sum and fall back to the highest-probability token (greedy):

let probSum = probsArray.reduce(0, +)
guard probSum > 0 else {
    return Int32(idxArray[0])  // topK is already sorted descending
}
let randomValue = Float.random(in: 0..<probSum, using: &rng)