- Core Header proposal moved to a linear async function outside the Core loop (see `Core::propose_header`)
- Responding to vote requests moved to primary RPC handler outside the Core loop (see `PrimaryReceiverHandler::request_vote`)

Notes:
- Header-handling tests in `core_tests.rs` largely replaced by equivalent tests in `primary_tests.rs`
- New header proposal tests in `core_tests.rs`
- Deletes changes from PR #5793, which are now driven by retries of vote requests from remote primaries