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>

Author: ruvnet

v2/crates/wifi-densepose-bfld/src/privacy_mode.rs

@@ -132,6 +132,10 @@ pub struct PrivacyAttestationProof {
     pub hash: [u8; 32],
 }
 
+// `compute` is only reachable through `PrivacyModeRegistry` (the std-gated
+// audit log); without `std` there is no caller, so gate it to match and avoid
+// a dead-code error under `--no-default-features` + `-D warnings`.
+#[cfg(feature = "std")]
 impl PrivacyAttestationProof {
     fn compute(mode: PrivacyMode, prev_hash: [u8; 32]) -> Self {
         let action_bits = mode.action_bits();