feat(cuts): cut-lens v2 — better, still not trustworthy, and now we know why

Second attempt at locating crossfaded track boundaries without PLN's ears, with
the three faults of v1 fixed: references taken ADJACENT to the boundary instead
of from a track midpoint (a livecoded track is not stationary), a MEDIAN over
every separating orbit instead of a weighted mean over 1-5 (one liar cannot move
a median), and a CUSUM change-point instead of a threshold plus a 6s sustained-
crossing rule that could not resolve a 4-cycle crossfade (~7.7s at 124 BPM).

It got better: median error 24.0s -> 16.6s, voters 1-5 -> 4-8. It is still not
good enough to ship, and I did not ship it.

The useful part is the SHAPE of the failure. Every single error is negative, and
the accurate boundaries are exactly the ones PLN described as short:

    #3 -21.0  #4 -15.2  #5 -18.0  #6 -18.5  #7 -41.2   (his: 'still the drops')
    #8  -4.4  #9  -2.0  #10 -2.4                        (his: 'perfect starts perfect')

The lens finds where the incoming track FIRST APPEARS; the ear marks where it
TAKES OVER. They differ by the length of the crossfade, which is precisely the
quantity that varies. So the lens is measuring a real thing, just not the one we
asked for — a much better position than 'it is noisy'.

Two ways forward are written into the file, plus an explicit warning not to
paper over it with a constant offset fitted to these eight points: the errors
span 2.0-41.2s, so a fitted constant would be memorisation, not a method.

Prior art checked at PLN's suggestion and worth recording: the OPAL gig log is
useless here (2 track events in 87 minutes — the gig-log 'track' field only
started recording properly in 97b22c0b, AFTER the gig), but
Prod/Montreuil26_master/boundaries_ground_truth.json holds 14 user-confirmed
boundaries from a RELEASED split. That is a second, independent labelled set the
next attempt should validate against before touching OPAL.

armada/tide-table/judge_specs/opal26.json

@@ -10,7 +10,6 @@
   "title": "Sunset Forest",
   "date": "2026-08-08",
   "variant": "stream",
-
   "liveRoot": "/home/pln/Work/Sound/Tidal",
   "clipsDir": "/home/pln/Work/Sound/Prod/Opal26_master/heads_v4",
   "tracksDir": "/home/pln/Work/Sound/Prod/Opal26_master/tracks_v4_streaming",
@@ -19,13 +18,20 @@
   "stemsGlob": "*.wav",
   "stemmap": "/home/pln/Work/Sound/Tidal/armada/tide-table/stemmap_opal26.json",
   "tracksJson": "/home/pln/Work/Web/www/content/lives/2026/opal-festival-2026/tracks.json",
-
   "audioBase": "/audio/heads",
   "fullAudioBase": "/audio/full",
-
-  "keeps": [[6.0, 4540.0], [4569.9, 4822.27]],
+  "keeps": [
+    [
+      6.0,
+      4540.0
+    ],
+    [
+      4569.9,
+      4822.27
+    ]
+  ],
   "binS": 2.0,
-
   "calibration": "clip time = master time; master → stem via keeps [[6.0,4540.0],[4569.9,4822.27]] (sums to 4786.370 s = the master to the millisecond)",
-  "note": "v4 = the premix pass that trimmed d4, which owned 76-91% of the low-frequency bed floor. Per-track loudness deliberately keeps the live arc: tracks range roughly -12.5 to -18.2 LUFS rather than being flattened to a single number."
+  "note": "v4 = the premix pass that trimmed d4, which owned 76-91% of the low-frequency bed floor. Per-track loudness deliberately keeps the live arc: tracks range roughly -12.5 to -18.2 LUFS rather than being flattened to a single number.",
+  "segments": "/home/pln/Work/Sound/Prod/Opal26_master/segments_v3.json"
 }