fix(bfld): gate PrivacyAttestationProof::compute behind std

CI `cargo test --no-default-features (baseline regression)` failed with
`error: associated function compute is never used` under -D warnings.
compute() is only reachable via PrivacyModeRegistry (#[cfg(feature =
"std")]); without std there is no caller. Gate the impl to match its only
callers. Verified clean under --no-default-features, default, and
--features mqtt with RUSTFLAGS=-D warnings.

Co-Authored-By: claude-flow <ruv@ruv.net>