docs(training-playbook): 4.6.2 — a null result needs a positive control

From the run-2 gate. A memorisation probe reporting 0.00% across all 72 items is
the correct output for a model that has not seen the corpus, and is also the
exact output of a probe that is not firing. Nothing in the number distinguishes
them. brokkr-smithy-dev drove the overlap function with known-answer inputs
(identical 100%, half-verbatim 65.38%, unrelated 0%, empty 0%) before trusting
the null, which is what converts a suspicious zero into evidence.

This is 4.5's inert gate wearing a different face: there a check that could not
return 'fail', here a measurement that cannot return non-zero. A clean null is
the most reassuring output an instrument produces and the least
self-evidencing.

Same section records the identical-on-both-arms variant: the diversity battery's
rp family froze zero markers, so its attractor hit rate read 0.0 on base AND
tuned. That reads as a clean result and means the instrument cannot discriminate
on that family. Report as a bounded limitation, never as a delta of zero -- a
check returning the same value for every input is not measuring.

Checklist gains the line.

docs/pfi/training-throughput-playbook.md

@@ -861,6 +861,38 @@ that, because the instrument was dark.
 
 If breaking it changes nothing, you have documentation, not a test.
 
+#### ⚠ 4.6.2 ⭐⭐ A NULL RESULT needs a positive control before it counts as a null
+
+`0.00% / floor 0.00% / max_item 0.0%` across all 72 items is the correct output
+of a memorisation probe on a model that has never seen the corpus. **It is also
+the exact output of a probe that is not firing at all**, and nothing in the
+number distinguishes them.
+
+The move that separates them costs one minute — drive the metric's own function
+with inputs whose answer you already know:
+
+    identical text        100.00%
+    half-verbatim          65.38%
+    unrelated English       0.00%
+    empty string            0.00%
+
+Now the zero means something: the instrument *can* go red, and did not.
+
+⚠ **This is §4.5's inert gate wearing a different face.** There it was a check
+that could not return "fail"; here it is a measurement that cannot return
+non-zero. A clean null is the most reassuring output any instrument produces and
+the least self-evidencing, so it is precisely the one that has to be earned.
+
+Same trap in a metric that reads **identical on both arms**: a diversity battery
+whose rp family froze zero markers reported an attractor hit rate of 0.0 for
+base *and* tuned. That reads as "no attractors, clean result" and means "this
+instrument cannot discriminate on this family." Report it as a bounded
+limitation — that family is measured on one axis rather than two — never as a
+delta of zero. **A check that returns the same value for every input is not
+measuring.**
+
+(Both from the run-2 gate, brokkr-smithy-dev, 2026-08-26.)
+
 #### ⚠ 4.6.1 …but calibrate the gate against a CORRECT result, not a convenient one
 
 The opposite failure, and it costs trust rather than correctness. A coherence
@@ -949,6 +981,9 @@ completed, plausible, wrong run above.
 [ ] log rotates on relaunch       one file describes one run
 [ ] present-and-null, not absent  a run that claims nothing must say so explicitly
 [ ] watchdog tested negative      kill something and confirm it fires
+[ ] every NULL has a positive     drive the metric with known-answer inputs;
+    control                       a clean zero is the least self-evidencing
+                                  result any instrument produces
 [ ] stale numbers grepped         for the figure, repo-wide, not just in view
 ```