Merge the scene model: a second, ADE20K-trained graph for per-category grades

Five commits. `models/` becomes one tree at the repository root so the
weight the application carries is a single `du -sh`; the export script
stops building its multi-gigabyte venv in RAM; `yolo26s-sem-ade20k`
joins the instance model rather than replacing it; the decoder turns its
logits into a partition of unity over eight photographic categories; and
four packaging routes put the file somewhere each platform can find it.

The instance model stays exactly where it was. A semantic model merges
every pixel of a class into one region, so it cannot separate two people,
and separating two people is what clicking a subject needs. The scene tab
grades whole categories and does not care. docs/segmentation.md §16.

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

# Conflicts:
#	docs/traceability.md

ui/dr-ui/src/lib.rs

@@ -73,6 +73,15 @@ pub use develop::DevelopSession;
 /// opened — see `library::shared_face_models_dir`.
 pub use library::shared_face_models_dir;
 
+/// The scene model's three files, wherever this device keeps them.
+///
+/// Public for the same reason as the directory above: the pieces that reach for
+/// a model are not all inside this crate. Exported ahead of the scene tab that
+/// will consume it so that the packaging and unpacking added alongside it have
+/// something to be verified against — `assemble-apk.sh` writing files no lookup
+/// looks for would be a silent mistake for as long as the tab took to arrive.
+pub use library::scene_model;
+
 pub mod launch;
 pub mod launch_ui;