refactor: harden facet runtime boundaries and macro support surface

[darinkishore]

Summary

• remove remaining runtime compat-trait usage from typed runtime paths and rely on focused helper APIs
• add FieldCodecRegisterContext and pass contextual metadata through facet adapter registration hooks
• harden dspy-rs exports by routing macro internals through hidden __macro_support paths
• update macro expansion and callsites/tests to follow the narrowed boundary
• split unrelated docs draft work into a separate sibling WIP commit (excluded from this PR)

Validation

• cargo fmt --all
• cargo check --workspace
• cargo test --workspace