Drop "half" from _VALID_DTYPE_OVERRIDES / _LOW_PRECISION_DTYPES.
get_dtype() has never accepted "half", so VOXCPM_MPS_DTYPE=half would
pass override validation and then crash downstream with
"Unsupported dtype: half". The remaining aliases (bfloat16/bf16,
float16/fp16, float32/fp32) already cover the intended dtype space.
Adds a standalone unit check under scripts/ to guard the invariant
that every accepted override parses through get_dtype().
Addresses review feedback on #263.
Oumnya