feat: implement --stop-at/--stop-after deadline enforcement

[oferchen]

Summary

• Add DeadlineChecker in crates/transfer/src/shared/deadline.rs that checks Instant::now() against a pre-computed deadline
• Wire stop_at / stop_after config through ServerConfig to generator and receiver transfer loops
• Both generator and receiver check deadline before processing each file, returning TransferDeadlineReached error on expiry
• Add unit tests for deadline config parsing and enforcement behavior

Test plan

• All 22,048 existing tests pass
• cargo fmt clean
• cargo clippy clean
• CI: fmt+clippy, nextest (stable), Windows, macOS, Linux musl