Offer three denoise networks and a method to choose between them

AI Denoise's Apply switch becomes Method: Bilinear, Fast, Medium, Best,
default Best, so an untouched raw writes nothing and develops through the
mixture. `apply` is still read and never written: 0 is Bilinear, 1 keeps
a network already chosen.

- Best is the mixture of a flat and an edge expert with a learned gate;
  Medium and Fast are students distilled from it. 2.48 s, 0.79 s and
  0.57 s for a 20 MP frame on TensorRT fp16.
- Each network carries its own tile border (256 for the mixture, 192 for
  the students) through `dr_denoise::Shipped` and `TileNet::halo`.
- The file is hashed once at open and each network keys its own cached
  result; Bilinear keeps the result in memory for the way back.
- Each has an .a16w16 sibling for the Hexagon: 0.00 dB on the 6D gate,
  at most 0.11 dB with the noise scaled x0.5 to x4.
- APK BUNDLED 19 -> 23; the PKGBUILD installs all three.

packaging/PKGBUILD

@@ -124,12 +124,15 @@ package() {
     fi
     install -Dm644 "${_src}" "${pkgdir}/usr/share/darkroom/models/migan-512.onnx"
 
-    # The learned demosaic and denoise (the project's own weights, GPL —
-    # models/LICENCE.md). Same pointer check, same directory.
-    _src="models/denoise/mosaic-1408.onnx"
-    if [[ "$(stat -c%s "${_src}")" -lt 100000 ]]; then
-        echo "error: the denoise model is an LFS pointer — run: git lfs pull" >&2
-        return 1
-    fi
-    install -Dm644 "${_src}" "${pkgdir}/usr/share/darkroom/models/mosaic-1408.onnx"
+    # The learned demosaic and denoise, one network per method (the project's
+    # own weights, GPL — models/LICENCE.md). Same pointer check, same
+    # directory.
+    for _net in fast medium best; do
+        _src="models/denoise/mosaic-${_net}-1408.onnx"
+        if [[ "$(stat -c%s "${_src}")" -lt 100000 ]]; then
+            echo "error: the ${_net} denoise model is an LFS pointer — run: git lfs pull" >&2
+            return 1
+        fi
+        install -Dm644 "${_src}" "${pkgdir}/usr/share/darkroom/models/mosaic-${_net}-1408.onnx"
+    done
 }