probe: mark the (S - D) median when it is below its own resolution

Fifth instance of the defect class this PR keeps turning up — a number printed
without the fact that the measurement cannot support it. Observed on the run
that prompted this commit: the line printed `1.33 ns` against a noise floor of
`1.57 ns`, i.e. a value indistinguishable from zero, as a bare figure, while
every per-width figure on the screen above it already carries `~n` in exactly
that situation.

The line now carries the same `~noise` marker under the same condition
(`med_gap.abs() < floor`).

Worth stating as the general form, since one instance of this class was fixed
three times in this PR before the class itself was named: finding a mislabelled
statistic is not the same as finding the mislabelled statistics, and the cheap
sweep — read every print against what its accumulator actually holds — is what
catches the rest.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DCfrD5y19cvFc4AoyydXYv

Author: claude

examples/mask_algebra_tail_probe.rs

@@ -532,9 +532,15 @@ mod imp {
         // defect class as the pooling bug one screen up (a label outrunning what
         // the code computes) and a reminder that finding one instance of that
         // is not the same as finding them all.
+        // ...and mark it when it is below its own resolution, as every other
+        // figure here is. Measured on the run that prompted this: 1.33 ns
+        // against a 1.57 ns floor — a bare number there is the same defect one
+        // more time, a value printed without the fact that it cannot be
+        // distinguished from zero.
         println!(
             "median (S - D), pooled over all {REPEATS} passes and ALL widths (unfiltered): \
-             {med_gap:.2} ns   [noise floor {floor:.2} ns]"
+             {med_gap:.2} ns{}   [noise floor {floor:.2} ns]",
+            if med_gap.abs() < floor { "  ~noise" } else { "" }
         );
 
         let lo = |v: &[f64]| {