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.

core/dr-denoise/src/lib.rs

@@ -27,6 +27,33 @@ use dr_decode::RawImage;
 pub use noise::{NoiseModel, Source};
 pub use tile::{TileNet, HALO};
 
+/// TRACES: FR-DEV-3g
+/// A network the app ships in `models/denoise/`: its file, and the context
+/// it needs past a tile's kept centre (docs/dev/denoise.md §13).
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct Shipped {
+    pub file: &'static str,
+    pub halo: usize,
+}
+
+/// The smallest student: 0.9 M parameters, 11 GMAC a megapixel.
+pub const FAST: Shipped = Shipped {
+    file: "mosaic-fast-1408.onnx",
+    halo: HALO,
+};
+/// A student of the mixture with the first release's shape: 3.2 M
+/// parameters, 48 GMAC a megapixel.
+pub const MEDIUM: Shipped = Shipped {
+    file: "mosaic-medium-1408.onnx",
+    halo: HALO,
+};
+/// The mixture: a flat expert, an edge expert and the gate that blends them.
+/// It reaches further than either, so it keeps a smaller centre of each tile.
+pub const BEST: Shipped = Shipped {
+    file: "mosaic-best-1408.onnx",
+    halo: 256,
+};
+
 #[derive(Debug, thiserror::Error)]
 pub enum DenoiseError {
     #[error("the network cannot take this photograph: {0}")]