Format the tree the way the gate asks for it

`cargo fmt --check` is a required step and had drifted across 45 files. Most of
it arrived this week: several operations were written in parallel worktrees and
merged by hand, and a hand-merge resolves conflicts without ever running the
formatter over the result.

No behaviour changes — this is `cargo fmt --all` and nothing else, kept as its
own commit so the next reader can skip it wholesale rather than search it for
one that matters.

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

core/dr-gpu/examples/segment.rs

@@ -12,8 +12,8 @@
 //! PPM for the same reason `develop` uses it: no encoder dependency, and
 //! every viewer reads it. This is a diagnostic, not an export path.
 
-use dr_segment::{MergeTree, RegionField};
 use dr_gpu::{DemosaicedImage, Demosaicer, GpuContext, SegmentOptions, SegmentPass};
+use dr_segment::{MergeTree, RegionField};
 
 /// The ladder the example dumps. Chosen to span "far too fine to be useful"
 /// through "one or two objects", because both ends are informative: if no rung