Let the fill show the model an open void: mirror depth 0 means no ring and nothing known beyond the band

docs/panorama.md

@@ -486,3 +486,40 @@ rectangle.
 `models/LICENCE.md`), installed by the PKGBUILD and unpacked by the APK
 beside the face and scene models; `tools/export-migan.sh` regenerates it
 from the upstream checkpoint.
+
+## 14. The fill, trained — 2026-09-20
+
+§13.5 named the remaining fault: in a deep corner the stock model puts its
+Places2 prior — clouds, peaks, a water line — into a hole, because it was
+trained on holes *inside* pictures and a panorama's border is a hole with
+the picture on one side and nothing on the other. Every ring (mirror,
+replicate, detrend) treated the symptom. The fix is a model that has seen
+the real thing: **MI-GAN's 512 generator fine-tuned on border-shaped voids
+cut from the user's own photographs**, in a separate repository
+(`darkroom-infill`, beside this one), so the truth beyond the void is known
+and the model learns one-sided extrapolation.
+
+**What it was trained on.** Voids made the way this merge makes them:
+frames with a yaw, a common pitch and per-frame roll, projected onto the
+cylinder and rasterised, the canvas their union's bounding box, the void
+the canvas outside the union — arcs where straight edges bent, cusps where
+frames meet, the bow-tie wedge at a corner (a third of tiles are cut at a
+canvas corner). Voids to 256 px deep at a 512 tile. Half the deep tiles
+train the *second pass*: a no-grad first pass fills the tile, its nearest
+band (64–256 px) is marked known, and the remaining void is the example —
+so the model continues its own output without drift, which is how `fill`
+runs deep voids. Data: ~7 400 pictures — the 1024-px proxy tier of the
+library and ~2 000 raws sampled evenly across every year, developed at
+half size. Loss: hole-weighted L1, VGG16 perceptual, a hinge PatchGAN.
+One night on the reference desktop's RTX 3050.
+
+**What changed here.** `FillParams::mirror_depth` **0** is now "no ring":
+the void reaches the tile's edge with nothing beyond, and beyond the band
+being filled the void stays *unknown* rather than presented as known coarse
+fill — the two conditions the model was trained under. Defaults: mirror 0,
+coarse 1 (the coarse pass seeds nothing the model is allowed to see), band
+192. The ring remains on the page for the stock model's sake, at any depth
+above zero. The model file is a drop-in (`models/inpaint/migan-512.onnx`,
+same six operators, same tensors) and the engine loads it unchanged.
+
+**Measured.** PLACEHOLDER