Test that a stub decoder reaches the scan, the ladder and export

FR-RAW-2's "without changing callers" needs a test that would fail if a
caller named the concrete decoder; passing a real RAW through rawler
cannot tell the two apart, because both routes give the same answer.

The decoder_seam tests hand a stub decoder, for a container no real
decoder reads, to the catalog scan (read_metadata_only over a folder
backend), the preview ladder (the remote two-stage fetch, an import's
thumbnail and the viewer's no-GPU fallback) and export (open_for_export,
skipped without an adapter). Each assertion is on something only the
stub produces: its camera and date, a header fetched at its 64-byte
budget rather than HEADER_BYTES, preview and sensor sizes turned by its
orientation. Switching collect_metadata or make_thumbnail back to the
free functions fails two of the three tests.

The develop test_support module is widened to the crate so the export
test shares the one headless GPU context the other tests use. The
requirements note for FR-RAW-2 now records the trait as built and the
second decoder as not.

ui/dr-ui/src/develop/mod.rs

@@ -48,7 +48,7 @@ pub(super) const SEGMENT_PROXY_EDGE: u32 = 1600;
 /// the code they exercise left these three needed in most of the resulting
 /// files, so they live here once instead of being copied.
 #[cfg(test)]
-pub(super) mod test_support {
+pub(crate) mod test_support {
     use super::*;
     use dr_gpu::GpuContext;
 
@@ -142,7 +142,7 @@ pub(super) mod test_support {
     /// `OnceLock` rather than `lazy_static`: the initialiser runs once however
     /// many threads arrive together, and the losers block until it is done —
     /// which is precisely the property that was missing.
-    pub(super) fn headless() -> Option<GpuContext> {
+    pub(crate) fn headless() -> Option<GpuContext> {
         static SHARED: std::sync::OnceLock<Option<GpuContext>> = std::sync::OnceLock::new();
         SHARED
             .get_or_init(|| pollster::block_on(dr_gpu::GpuContext::new_headless()).ok())