Simplify structured output metadata

[waywardmonkeys]

Summary

• move universal ID metadata out of semantic Value variants into the VM value arena
• keep host operands and dynamic options free of output-only metadata
• replace parallel forwarding sinks with one metadata-aware adapter
• add ID precedence regression coverage and a structured-output benchmark

Size

Runtime code is 147 additions and 266 deletions versus main: net -119 lines. Including tests and the benchmark, the PR is net -58 lines.

Performance

Back-to-back Criterion runs of a stored identified number rendered three times measured:

• main: 501.5 ns
• this PR: 466.8 ns
• improvement: about 6.9%

Ordinary formatting benchmarks remained effectively neutral within run-to-run noise.

Validation

• typos
• cargo fmt --all -- --check
• taplo fmt --check on tracked TOML files
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo test --workspace --all-features
• cargo doc --workspace --all-features --no-deps
• no-default, compile-only, and ICU4X feature checks
• WG scoreboard: 462/462