Reorganize the MMP subsystem, behavior unchanged throughout:
- Restore the pre-migration role split: move SenderState into sender.rs,
ReceiverState (with its GapTracker helper) into receiver.rs, MmpMetrics and
RrLog into metrics.rs, and PathMtuState into path_mtu.rs, leaving the Mmp
aggregate plus the peer/session state in state.rs (1339 to 196 lines). Home
the module constants in a new limits.rs and re-export them at the same paths.
Pure code-motion with module rewiring; visibility unchanged.
- Dissolve the 97-line src/mmp shell, which held only MmpConfig and the
monotonic mono_ms() clock: move MmpConfig into config/node.rs (re-exported as
crate::config::MmpConfig), move mono_ms() into a new top-level src/time.rs
shell time seam, repoint all callers, and delete src/mmp/. Also correct stale
src/mmp/ doc-comment path labels to proto/mmp/ where they name the protocol
primitives.
