Shared PMF quantization + validation primitives. Used by PerGroupCodebook (PGC, alphabet=16, total=256) and Order1ScaleAC (alphabet=256, total=4096); each codec passes its own `total` constant.

Quantize raw integer counts into u16 counts summing to exactly `total`. Applies Laplace +1 smoothing first (every entry becomes ≥1, guaranteeing no zero-prob symbol on encode/decode).

Verify that quantized counts sum to exactly `expected_total` AND that every entry is at least 1. Returns a `CodecDecode` error on mismatch (state-load corruption tripwire). Zero-probability symbols are forbidden because the range coder divides by `prob` on encode and would loop forever on `prob == 0` during decode renormalization.