Tidy what the seven-branch merge left behind

Three lints, all from merged work rather than from any one branch:
`terrace` and `disc` were steps on the way to the ramp the plateau test now
uses, and the reasoning that discarded them lives in docs/segmentation.md §12
rather than needing the code; two mechanical clippy suggestions in segment and
cache.

1176 tests pass, clippy and fmt clean, traceability regenerated.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

core/dr-catalog/src/cache.rs

@@ -709,7 +709,7 @@ mod tests {
         // the whole trip back down.
         let (cat, cache, _dir, ids) = fixture(1);
         cache
-            .store(cat.connection(), ids[0], "a.CR2", &vec![0u8; 100], true, 10)
+            .store(cat.connection(), ids[0], "a.CR2", &[0u8; 100], true, 10)
             .unwrap();
 
         cache.release(cat.connection(), &ids).unwrap();