Get the scene model onto the devices that need it

The decoder can load from a path; nothing yet put a file at one. Four
packaging routes, and one lookup that finds the result.

## Not `include_bytes!`, unlike the instance model

The instance model is 11 MB and compiled in, which was the right call for
it: Android hands the app no filesystem path (ARCH §6.9) and 11 MB is
tolerable. The scene model is 24 MB, and 35 MB of constants in the binary
is paid by every install whether or not the tab is ever opened.

So it follows `models/face/` instead — carried as an APK asset, unpacked
once at first launch into the shared directory a desktop install already
uses, after which every lookup finds it where it finds a desktop user's.
Assets are stored rather than deflated in the APK, so unpacking is a copy
rather than an inflate.

`embedded-scene-model` exists for the desktop build with nowhere else to
read from, and for tests wanting the real graph. Off by default, which is
the asymmetry with `embedded-model` and the reason for a separate
feature.

## Three files, all or none

`scene_model` insists on the graph, its vocabulary and the category
descriptor together, for the reason `face_models` insists on its pair: a
graph alone decodes to 150 anonymous channels. Reporting the set missing
beats starting and failing at the first inference.

## The two model sets are not the same kind of thing

`install_bundled_models` now carries both, and the distinction is worth
keeping in view. Face weights are absent from the repository *by design*
— the InsightFace grant is research-only (docs/faces.md §2) — so a build
carrying none is ordinary. The scene model is committed, so a build
carrying none means a checkout without `git lfs pull`.

Neither is fatal. A photo editor that refuses to start over a missing
grading feature is worse than one that starts without it, so both report
themselves unavailable exactly as face indexing already did.

The LFS-pointer guards apply to the `.onnx` only. The vocabulary and the
descriptor are legitimately a few kilobytes, and a size check that fails
on them would be a guard against the wrong thing.

`scene_model` is exported ahead of the tab that will consume it so the
packaging added here has something to be verified against — assets
written where no lookup looks would be a silent mistake for as long as
the tab took to arrive.

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

packaging/flatpak/paris.tourolle.darkroom.yml

@@ -165,6 +165,21 @@ modules:
           install -Dm644 "models/face/$m" "/app/share/darkroom/models/$m"
         done
 
+      # The scene model, for the per-category grades. In the repository, unlike
+      # the face weights — AGPL, and GPLv3 §13 permits the combination
+      # (models/LICENCE.md) — so it installs unconditionally. Three files: the
+      # graph, its vocabulary, and the category descriptor; dr_ui reports the
+      # tab unavailable without any one of them. The pointer check is on the
+      # graph alone, the other two being legitimately small.
+      - |
+        if [ "$(stat -c%s models/scene/yolo26s-sem-ade20k.onnx)" -lt 100000 ]; then
+          echo "error: the scene model is an LFS pointer — run: git lfs pull" >&2
+          exit 1
+        fi
+        for m in yolo26s-sem-ade20k.onnx yolo26s-sem-ade20k.classes.json categories.txt; do
+          install -Dm644 "models/scene/$m" "/app/share/darkroom/models/$m"
+        done
+
       - install -Dm644 README.md /app/share/doc/darkroom/README.md
 
     sources: