Summary

Implement Toyota Way / Poka-yoke validation in AprWriter to prevent malformed models from being written.

Background

During WAPR-TRANS-001, a missing Slaney-normalized filterbank caused 89.4% mel divergence (sign flip). The model was created without embedded filterbank, falling back to broken computation.

Requirements

Gate Checks (Jidoka - stop the line)

1. Filterbank Required: If model uses mel spectrogram, filterbank MUST be embedded
2. Filterbank Scale Check: filterbank.max() < 0.1 (Slaney-normalized range)
3. Smoke Test: Silence input must produce negative mel mean

API

impl AprWriter {
    /// Validate model before write. Returns error if gates fail.
    pub fn validate(&self) -> Result<(), ValidationError>;
    
    /// Write with mandatory validation (Jidoka)
    pub fn write_validated(&self) -> Result<Vec<u8>, WhisperError>;
}

Acceptance Criteria

• AprWriter::write() calls validate() by default
• Clear error messages pointing to fix
• Unit tests for each gate
• Documentation in spec

References

• APR-VERIFY-001 Case Study (Section 11)
• Toyota Production System: Poka-yoke (mistake-proofing)