fix(utils): saturate quantize output to avoid sign-flip wraparound

[dlyongemallo]

Fixes #49.

[Copilot]

Pull request overview

This PR fixes a signed wraparound bug in utils.quantize that caused full-scale positive samples (e.g., +1.0) to overflow the representable signed integer range and decode with a sign flip (impacting QSM/MQSM), by saturating/clipping instead of wrapping.

Changes:

• Add saturation (via clipping) to quantize to keep outputs within the signed integer range for the given qubit_depth.
• Add input validation for qubit_depth (must be an integer ≥ 1; accepts NumPy integer scalars but rejects bools).
• Add focused unit tests covering in-range behavior, positive/negative saturation, and qubit_depth validation.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
quantumaudio/utils/convert.py	Clamp quantization outputs to the representable signed range and validate qubit_depth to prevent overflow-induced sign flips.
tests/test_convert.py	New unit tests validating saturation behavior and qubit_depth type/range handling.

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