fix: use subtle for constant-time comparison

[nanookclaw]

Summary

Replace Daphne's hand-rolled constant_time_eq() helper with subtle::ConstantTimeEq, which is already available through the workspace and daphne crate dependencies. The public helper keeps the same &[u8] -> bool API, so existing bearer-token and request-body comparison call sites do not need to change.

This also addresses the related Prio3 count TODO by converting valid count measurements to bool with ConstantTimeEq before sharding. Non-binary count measurements still follow the existing rejection path.

Tests

• cargo fmt --check
• cargo test -p daphne constant_time_eq_matches_slice_equality
• cargo test -p daphne count_rejects_non_binary_measurement
• cargo test -p daphne --lib
• git diff --check

Closes #247.