Reject incomplete init_mixw models

[lenzo-ka]

Defect

When a destination base phone is absent from the source CI model, uniform mixture-weight initialization does not initialize the corresponding Gaussian codebook. The existing completeness checks only warn, after which init_mixw writes all destination files and returns success even though the acoustic-model initialization invariant is not satisfied.

Fix

Record any uninitialized destination state or codebook, emit an error naming each incomplete index, and return failure before the first destination write. The existing transition-matrix fallback remains unchanged because it provides a complete initialization policy. Complete inputs follow the same path and produce identical output.

Verification

• cmake -S . -B build
• cmake --build build

This mirrors the landed completeness gate in pstrain PR #127.