Never crash: gpu and merge free of panicking calls

gpu (its render already runs under catch_unwind and falls back to the CPU):
- a failed readback (device lost, out of memory) logs, marks the GPU
  broken and returns zeros, so crate::render discards the result and
  renders on the CPU, instead of panicking;
- an unknown kernel name does the same instead of panic!;
- finish-parameter fields are looked up as Option (debug_assert on an
  unknown name) instead of panic!;
- the warp sampler matches (mode, warp) like the CPU pipeline;
- Buf::raw keeps one documented expect (Some from construction to Drop).

merge:
- pyramid building (push_pull, Blender::finish), camera graph growth,
  bundle adjustment, focal median and alignment use let-else / filter_map
  instead of expect;
- synth::bracket_dngs / pano_views return Result (a missing demo scene
  or a DNG write error is an error); lightcraft-cli synth-merge and the
  engine merge tests handle it.

lightcraft-gpu, lightcraft-merge and lightcraft-cli now #![deny]
unwrap/expect/panic/unimplemented/todo/unreachable; clippy.toml as in the
steering PR.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/tests_merge.rs

@@ -21,7 +21,7 @@ fn import(s: &mut Session, paths: Vec<String>) -> Vec<u64> {
 fn hdr_merge_command_creates_and_imports_a_dng() {
     let dir = temp_dir("hdr");
     let mut paths = Vec::new();
-    for (i, b) in lightcraft_merge::synth::bracket_dngs(480, 320, &[-2.0, 0.0, 2.0]).into_iter().enumerate() {
+    for (i, b) in lightcraft_merge::synth::bracket_dngs(480, 320, &[-2.0, 0.0, 2.0]).unwrap().into_iter().enumerate() {
         let p = dir.join(format!("IMG_{i}.dng"));
         std::fs::write(&p, b).unwrap();
         paths.push(p.to_string_lossy().to_string());
@@ -71,7 +71,7 @@ fn hdr_merge_command_creates_and_imports_a_dng() {
 fn panorama_command_with_auto_crop() {
     let dir = temp_dir("pano");
     let mut paths = Vec::new();
-    for (i, v) in lightcraft_merge::synth::pano_views(480, 360, 420.0, &[-25.0, 0.0, 25.0]).into_iter().enumerate() {
+    for (i, v) in lightcraft_merge::synth::pano_views(480, 360, 420.0, &[-25.0, 0.0, 25.0]).unwrap().into_iter().enumerate() {
         let img = v.to_srgb8();
         let png = lightcraft_codecs::encode_png(&lightcraft_codecs::EncodeImage::rgba8(&img), &lightcraft_codecs::EncodeMeta::default()).unwrap();
         let p = dir.join(format!("P_{i}.png"));