Give the UDP microbench a reason and record the retry asymmetry

The UDP recv microbenchmark carried a bare #[ignore] while its sibling in
the link module carries a self-describing one. Match it, so the reason
appears wherever the test is listed rather than only in the doc comment.

Record why [profile.ci] retries = 2 is not applied locally. The two gates
disagree on purpose: the hosted runner retries a flaky test twice, the
local sweep fails on the first failure. It exists for shared-runner packet
loss, a property of that environment rather than of the code, and applying
it locally would suppress a real local flake — a failure that only
reproduces under load is a robustness bug to fix, not to retry past.
Keeping the local sweep strict is what makes it the sharper gate. An
undocumented asymmetry is indistinguishable from an oversight, which is
why this is written down rather than left to be rediscovered.

The cost is stated rather than hidden: a test that fails once and passes
on retry is reported green with no separate signal, so a genuine
intermittent failure can be absorbed. If that starts mattering the fix is
to surface retried-but-passed tests, not to drop the retries.

Quartet clean: fmt, build --workspace, clippy --all-targets -D warnings,
and test --lib at 1376 passed / 0 failed / 7 ignored.

.config/nextest.toml

@@ -5,6 +5,23 @@ junit = { path = "junit.xml" }
 # occasional msg1 under burst load even with the per-edge repair loop.
 # Allow a retry rather than failing the whole CI run on a single
 # dropped packet.
+#
+# Deliberately scoped to [profile.ci] and NOT applied locally, which makes
+# the two gates disagree: the hosted runner retries a flaky test twice, the
+# local sweep fails on the first failure. The asymmetry is intended and this
+# is the record of why, since an undocumented one is indistinguishable from
+# an oversight.
+#
+# It is here for shared-runner packet loss, a property of the hosted
+# environment and not of the code. Applying it locally would suppress a real
+# local flake, and a failure that only reproduces under load is a robustness
+# bug to fix rather than to retry past. Keeping the local sweep strict is
+# what makes it the sharper of the two gates.
+#
+# The cost, stated rather than hidden: a test that fails once and passes on
+# retry is reported green here with no separate signal, so a genuine
+# intermittent failure can be absorbed. If that starts mattering, the fix is
+# to surface retried-but-passed tests, not to drop the retries.
 retries = 2
 
 [test-groups]