feat(output): harmonize canonical schema for 0.4.2

[maebli]

Summary

• advance canonical output to schema version 2
• standardize frame functions, record functions, quantities, data coding names, and unit strings across Rust, Python, WebAssembly, JSON, YAML, CSV, tables, Mermaid, and annotations
• expose standalone DIF/VIF record decoding through the canonical record contract
• serialize annotation identifiers as snake_case
• document the naming and interoperability contract
• prepare Rust, CLI, Python, and WASM package metadata for release 0.4.2

Why

Output surfaces previously relied on a mixture of Rust Debug/Display representations and binding-specific record serialization. Units were represented as a nested object with both display and UCUM fields, while standalone Python record decoding returned a different structure from complete-frame decoding. This made equivalent data differ by output format and runtime.

Impact

Canonical structured output now uses schema version 2. The unit field is a single standardized string when a mapping is available, with a readable fallback otherwise. Legacy compatibility formats and the libmbus-compatible XML vocabulary remain unchanged.

Release tags v0.4.2, cli-v0.4.2, python-v0.4.2, and wasm-v0.4.2 will publish the package set. The WASM release also rebuilds and commits the website artifacts under docs/.

Validation

• cargo test --workspace --all-features
• cargo clippy --all-features -- -D warnings
• Python extension API tests: 7 passed
• headless Firefox WASM tests: 8 passed
• cargo fmt --all -- --check
• git diff --check