Give the inference engine a denoiser role, kept off the Hexagon

The learned demosaic-and-denoise (denoise.md) runs through the engine like
every other model. fp16 cost it nothing measurable (0.00 dB at every ISO on
validation tiles), so it takes TensorRT's and MIGraphX's fp16 like the
detectors. int8 cost it 6 to 9 dB, far past a 0.5 dB gate, so the Hexagon
refuses the role outright rather than relying on no int8 sibling existing,
and the tablet runs it on the CPU.

core/dr-inference-engine/src/lib.rs

@@ -45,6 +45,11 @@ pub enum Role {
     /// convolutions, so any rung serves it; fp16 on TensorRT and int8 on
     /// the Hexagon are the point of it.
     Inpainter,
+    /// The learned demosaic and denoise on the raw mosaic (docs/dev/denoise.md).
+    /// fp16 costs it nothing measurable; int8 costs 6–9 dB, because 256
+    /// levels cannot hold the shadow steps it exists to recover — so the
+    /// Hexagon does not take it.
+    Denoiser,
 }
 
 /// Which numeric form of a model a session was built from.
@@ -116,9 +121,11 @@ impl Rung {
     /// Whether this rung runs `role` at all. The Hexagon takes int8 graphs
     /// only, and the embedder is never int8 (§7) — it runs on the CPU
     /// beside a detector on the NPU, so its vectors compare across devices.
+    /// Nor is the denoiser: its int8 form failed the 0.5 dB gate by 6–9 dB
+    /// (denoise.md §8), so it runs on the CPU there too.
     fn serves(self, role: Role) -> bool {
         match self {
-            Rung::Hexagon => role != Role::Embedder,
+            Rung::Hexagon => !matches!(role, Role::Embedder | Role::Denoiser),
             _ => true,
         }
     }
@@ -586,6 +593,7 @@ mod tests {
     #[test]
     fn the_hexagon_never_takes_the_embedder() {
         assert!(!Rung::Hexagon.serves(Role::Embedder));
+        assert!(!Rung::Hexagon.serves(Role::Denoiser));
         assert!(Rung::Hexagon.serves(Role::Detector));
         assert_eq!(Rung::Hexagon.form(Role::Detector), Form::Int8);
         // A detector offered in f32 on a Hexagon device lands on the CPU.