Fix WAV chunk ordering and RIFF padding

[AnthonyRonning]

Summary

• reject data chunks that precede the mandatory fmt chunk instead of emitting split WAV files without format metadata
• word-align odd-length data chunks and reserve their pad byte within the configured chunk-size limit
• use checked output sizing and RIFF u32 conversions before constructing each split file
• parse and validate every emitted chunk in regression tests, including odd optional chunks and valid 8-bit mono data

Format rule

RIFF/WAVE allows optional and unknown chunks around the mandatory chunks, but fmt must precede wave data. Appendix A of EBU Tech 3285 reproduces this Microsoft RIFF/WAVE ordering rule.

Validation

• nix develop -c cargo fmt --all -- --check
• nix develop -c cargo clippy --locked --all-targets --all-features -- -D warnings
• nix develop -c cargo test --locked --all-features (215 passed, 16 ignored)

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.