Offer the border fill on the merge page, experimental, with every knob on it

A Border choice beside the projection — crop to the picture, or fill it
— that redraws the preview filled so the invented pixels are seen before
they are confirmed (FR-MRG-1), greyed with the reason when the model is
not there. The job fills at half the composite's resolution in a
display-ish space (white balance, matrix, gamma; invertible) and samples
the result back into the linear DNG wherever no frame reached; the
sidecar's merge line says border filled and with which knobs.

Experimental because the fill is right in thin borders and wrong in deep
corners, where the model's Places2 prior puts clouds in sky and water
under grass; so its six knobs — working scale, edge erosion, coarse pass,
band width, mirror depth, seam feather — are sliders under the choice,
each committing a redraw, until the defaults are right.

ui/dr-ui/src/inference.rs

@@ -32,6 +32,9 @@ pub fn init(runtime_dirs: Vec<PathBuf>) {
         Role::EyeClassifier,
         dir.join(crate::library::SUNGLASSES_MODEL),
     ));
+    if let Some(p) = crate::library::inpaint_model() {
+        models.push((Role::Inpainter, p));
+    }
     models.retain(|(_, p)| p.is_file());
 
     dr_inference_engine::init(dr_inference_engine::Config {