Let the model say what a thing is and the watershed say where it ends

Local masking needs to know where an image's regions are. The watershed
spike (S15 arm A) found the boundaries but had no idea what any of them
enclosed; its coarse levels were geometric accidents. This adds the other
half and the thing that joins them.

`core/dr-segment` is where region reasoning now lives — the hierarchy moves
out of `dr-gpu`, which keeps only the pixel passes that are genuinely
shaders. The new crate is device-free and, without its default features,
model-free too: 20 of its tests need neither an adapter nor 11 MB of
weights.

Arm B runs YOLO26n-seg through `ort`. D13 framed inference as a choice
between `ort`'s C++ runtime and the pure-Rust dependency policy; that was a
false choice. `ort`'s `alternative-backend` feature unlinks the C entirely
and `ort-tract` supplies the API from tract, which is pure Rust. Measured
before committing to it: zero unsupported operators, 420 ms for 640x640,
and correct masks on bus.jpg. No NDK problem to solve, so D13's largest
tolerated exception is not needed.

Arm C is `prior.rs`, and it ships because the two arms fail in opposite
directions. Instance membership re-weights the merge saddles, so region
pairs the model believes share an object merge early and pairs straddling
its edge merge late. No boundary moves — only the order in which they
dissolve — which is how the result stays pixel-accurate at every level
while its coarse levels become named things.

Two things the spec assumed that turned out to be false, both recorded in
models/LICENCE.md: there is no usable ADE20K-trained YOLO, so the shipped
vocabulary is COCO's 80 subjects and *stuff* like sky and foliage must come
from arm A; and tract cannot parse a dynamic-shape export, so the graph's
input is fixed and tiling is the only route to more semantic resolution.

Weights are AGPL-3.0, which GPLv3 §13 permits and which makes the combined
work effectively AGPL. Deliberate, not accidental. They live in Git LFS,
and a build script fails with an instruction rather than embedding a
pointer file when the clone lacks them.

.gitattributes

@@ -0,0 +1,12 @@
+# Model weights live in LFS.
+#
+# `core/dr-segment/models/*.onnx` is ~11 MB of binary that changes wholesale
+# when it changes at all. In ordinary git objects every future revision of it
+# would be stored in full, in every clone, forever — and the one thing nobody
+# can do with it is a useful diff.
+#
+# Consequence worth knowing before it bites: a clone without git-lfs gets a
+# ~130-byte pointer file where the model should be. `dr-segment`'s build script
+# detects exactly that and fails with an instruction rather than embedding the
+# pointer and failing at inference time.
+*.onnx filter=lfs diff=lfs merge=lfs -text