Weigh the categories in the same precompute, and mask with them

The scene model shipped with a decoder and no caller. This runs it.

## Beside the instance pass, not instead of it

`compute` now does both on the same upright frame and lays both back down
the same way, so instance masks and category masks index into one grid —
the sensor's. A failure in the scene half is logged and dropped rather than
propagated: no scene model is an ordinary state, and a photograph that can
still be masked by subject should not become unopenable because the
categories are missing.

Categories under half a percent of the frame never reach the cache. A
control that does nothing when moved is worse than an absent one, and each
one it skips is a proxy-sized buffer not allocated.

## The shader needed nothing

A category reaches `dr-gpu` as a soft coverage buffer at proxy resolution,
turned into a distance field — which is exactly what a subject is. So they
share `MODE_SUBJECT`. That is not a shortcut taken for speed: the shader has
no way to tell them apart and no reason to want one. What differs is only
which model produced the coverage, and that has already happened by then.

Feather, falloff, dilation and erosion therefore work on a category on the
day it arrives, because they were never subject-specific.

## Where the weights come from

`scene-model` compiles the graph in and the desktop app takes it; Android
leaves it off and reads the copy `install_bundled_models` unpacks, because
24 MB of constant is worth avoiding in a mobile install and not worth the
plumbing to avoid on a desktop one. Embedded is tried first — a build that
has the weights compiled in should not be silently overridden by a stale
file in a data directory.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

ui/dr-ui/Cargo.toml

@@ -114,6 +114,15 @@ slint-build.workspace = true
 serde_norway.workspace = true
 
 [features]
+# Compile the scene model into the binary.
+#
+# On by default for the desktop app and *off* for Android, which unpacks the
+# same graph from APK assets instead — 24 MB of constant is worth avoiding in a
+# mobile install and not worth the plumbing to avoid on a desktop one. Without
+# it `segmentation::scene_categories` falls back to the installed-file lookup,
+# which is what a packaged desktop build uses too.
+scene-model = ["dr-segment/embedded-scene-model"]
+
 default = []
 # Debug convenience: re-read style.yaml at startup so a palette can be tuned
 # without rebuilding. Costs the constant-folding of every token, so it stays