Reject stale MMP receiver reports

Ignore duplicate or counter-regressed ReceiverReports before updating
RTT, loss, goodput, or ETX, so a delayed or reordered report can no
longer poison link metrics. Compute the RTT-from-echo sample with
checked timestamp arithmetic and reject zero, negative, or out-of-range
results instead of risking wrap or underflow on untrusted wire values.

On the sender side, when receiver dwell time overflows the u16 wire
field, suppress the timestamp echo (send 0) and saturate dwell to
u16::MAX rather than truncating, so a bogus small RTT cannot be formed.

Adds duplicate, out-of-order, wrapped-add, and future-dated (checked_sub)
sample tests, asserts loss and goodput stay unchanged on a dropped
duplicate, and covers the dwell-overflow echo suppression. Documents the
behavior in the MMP design note and CHANGELOG.

Co-authored-by: Johnathan Corgan <johnathan@corganlabs.com>

CHANGELOG.md

@@ -7,6 +7,13 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+
+- MMP sender metrics now ignore duplicate or regressed receiver reports
+  before updating RTT, loss, goodput, or ETX. Receiver reports also
+  suppress timestamp echo when dwell time overflows, so stale reports
+  cannot inflate SRTT.
+
 ### Added
 
 - `pool_inbound` and `pool_outbound` counters on the TCP and Tor