docs(armada): log 027 — the four findings that came from measuring the output

Periodicity measuring length then role, dead bars grading S, no headroom against amp=gain^4,
and 1 loop in 6 opening on a weak beat. Plus the two bugs the checks caught in my own work,
which is the argument for having them.

armada/tasks/027-the-label-and-the-lens.md

@@ -33,7 +33,8 @@ Then: run it, look at what came out, fix what was wrong, run it again.
 
 ## Prise (findings / artifacts)
 
-- `tools/foundry/engine/{stempack,roles,kitcheck}.py`, 18 new tests, suite 86 → 104.
+- `tools/foundry/engine/{stempack,roles,kitcheck}.py` + `kitgate.py` + `restage.py`,
+  26 new tests, suite 86 → 112.
 - MAREA declares its tempo in its filenames, so it is free ground truth. Final grid:
   **123.0 bpm against a declared 123 — 0.0 % off.**
 - **96 kHz was breaking the finder.** At native rate it returned 123/123/124/**119**/
@@ -48,6 +49,32 @@ Then: run it, look at what came out, fix what was wrong, run it again.
 - The probe was looking in the wrong place with the wrong lens, and threw away good
   material both ways — see below.
 
+### Then four more, found by measuring the output instead of reading it
+
+- **The periodicity check was measuring loop length.** Autocorrelation at lag *k* has
+  only *N−k* overlapping frames; normalised by `ac[0]` it scored a *perfectly periodic*
+  click track 0.418 / 0.623 / 0.754 at 2 / 4 / 8 bars. Then the floor had to go per role:
+  medians drums 0.754, fx 0.549, tonal 0.214, bass 0.173, **vox 0.087**. One threshold
+  condemned 12 of 20 vocal loops for the crime of being vocal.
+- **Dead bars grade S.** A 4-bar loop whose last two bars are a fade is silent-to-silent
+  — a perfect seam — at an exact bar length. Ten of the first 161 cuts were that. Now
+  rejected at export, not reported afterwards.
+- **Kits had no headroom.** MAREA's ALL DRUMS and KIT both peak at **+0.2 dBFS**, and
+  `amp = gain^4`, so `# gain 1.2` is +7.6 dB and the orbit clips the moment you touch it.
+  Kits now land at −6 dBFS as *one gain per kit* (per-file would flatten a sparse hat
+  against a full drum bus) and never boosting.
+- **1 loop in 6 opened on a weak beat.** The exact "timed ok but cut BCDA/DABC" complaint
+  from July. The roll point is zero-crossing snapped by up to ~10 ms against a 20 ms
+  attack window — enough to flip two near-tied slots. Rotation now checks its own result
+  and repeats: best slot 29 → 46 of 60, worse half 10 → **1**.
+
+And two bugs the *checks* caught in my own work, which is the argument for having them:
+`kitgate` re-grades the shipped files instead of trusting the manifest, and immediately
+found that staging applied its gain after grading, so the recorded number described audio
+that was never written. Separately, a report section I added never rendered — a
+`str.replace` whose anchor did not match, and the one patch in that file where I had not
+asserted on the anchor.
+
 ## Sel (the shareable learning)
 
 **Two ways to be wrong about a stem, and they pull in opposite directions.**
@@ -83,6 +110,14 @@ peak-picker finds peaks in a flat envelope — a steady 50 Hz sine yields 84 ons
 six seconds. HPSS was the obvious fix and is worse: it hears a modern kick as harmonic
 and calls a drum bus 9 % percussive.
 
+**And check the instrument before you accuse the rig.** The first measurement of "do
+loops start on their downbeat" said 17 %, which looked like a broken feature. It scored
+16 slots with no full-band term while the rotator scores `bars×4` slots *with* one — the
+disagreement was mine. Re-measured with the rotator's own scorer, factored out so the two
+cannot drift apart, the real number was 48 % best-slot with a median miss of 0.032: mostly
+near-ties, but a genuine 17 % starting on a weak beat. The bug was real *and* the first
+number was wrong, which is the annoying case.
+
 ## Hameçon (hook)
 
 "The machine told me the drum stem was a bass. It was right about every number and