Complete MF2 WG conformance coverage

[waywardmonkeys]

Summary

• correct percent selection, reannotation, and retained numeric options
• preserve function fallback diagnostics and host function availability
• tighten expression and function-option parsing
• expose structured formatting parts with bidi, fallback, markup, locale, field, and universal-ID metadata
• extend the WG harness for datetime parameters and structured-part expectations

The WG scoreboard improves from 415/462 on main to 462/462.

Validation

• cargo fmt --all --check
• taplo fmt --check on every tracked TOML file
• typos
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo test --workspace --all-features
• cargo doc --workspace --all-features --no-deps
• cargo check -p message-format --no-default-features
• cargo check -p message-format --no-default-features --features icu4x
• WG scoreboard: 462 passed, 0 failed

A fresh Astra review found no remaining Must, Should, or meaningful Could findings.