bcx 0.4.0

Bifrost Causal Exchange protocol primitives for signed causal meaning and proof composition.
Documentation
# Pull Request

## Summary

Describe what changed and why.

## Type

- [ ] Bug fix
- [ ] Feature
- [ ] Documentation
- [ ] Refactor
- [ ] Dependency update
- [ ] Security hardening

## Checklist

- [ ] I kept the change scoped to BCX's existing architecture.
- [ ] I updated docs or release notes when behavior changed.
- [ ] I added or updated tests for behavior changes.
- [ ] I ran `cargo fmt --all --check`.
- [ ] I ran `cargo clippy --workspace --all-targets -- -D warnings`.
- [ ] I ran `cargo test --workspace`.
- [ ] I checked dependency and license impact when adding or updating crates.
- [ ] I did not commit secrets, private keys, tokens, local runtime data, or
      generated artifacts.

## Security Notes

Describe any security-sensitive impact. Mention canonical encoding, signatures,
capabilities, replay handling, receipts, WHY traversal, transport bindings, or
dependency changes if they are touched.

## Follow-Up

List any known remaining work or intentionally deferred tasks.