chore(transcript): make sample_u64 advancing via sample bytes

[radik878]

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Update sample_u64 to draw 8 bytes from advancing sources (DefaultTranscript: sample(); FeltTranscript/QuadFeltTranscript: sample(8)), ensuring subsequent calls produce different values without changing bias handling or adding new checks.

What this fixes: Ensures sequential sample_u64 calls don’t repeat the same value unless the transcript input changes. Aligns behavior with rng-like expectations and with the STARK transcript’s advancing sampling pattern.