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 <[email protected]>

apps/lightcraft-cli/src/main.rs

@@ -10,6 +10,7 @@
 //! lightcraft-cli commands [--json]
 //! lightcraft-cli controls [--json]
 //! ```
+#![deny(clippy::unwrap_used, clippy::expect_used, clippy::panic, clippy::unimplemented, clippy::todo, clippy::unreachable)]
 
 mod alloc_release;
 
@@ -283,14 +284,18 @@ fn synth_merge(args: &[String]) -> Result<(), String> {
     let mut written = Vec::new();
     match kind {
         "hdr" => {
-            for (k, b) in lightcraft_merge::synth::bracket_dngs(1800, 1200, &[-2.0, 0.0, 2.0]).into_iter().enumerate() {
+            for (k, b) in lightcraft_merge::synth::bracket_dngs(1800, 1200, &[-2.0, 0.0, 2.0]).map_err(|e| e.to_string())?.into_iter().enumerate() {
                 let p = Path::new(&dir).join(format!("bracket-{k}.dng"));
                 std::fs::write(&p, b).map_err(|e| e.to_string())?;
                 written.push(p);
             }
         }
         "panorama" | "pano" => {
-            for (k, v) in lightcraft_merge::synth::pano_views(1200, 900, 1000.0, &[-50.0, -25.0, 0.0, 25.0, 50.0]).into_iter().enumerate() {
+            for (k, v) in lightcraft_merge::synth::pano_views(1200, 900, 1000.0, &[-50.0, -25.0, 0.0, 25.0, 50.0])
+                .map_err(|e| e.to_string())?
+                .into_iter()
+                .enumerate()
+            {
                 let p = Path::new(&dir).join(format!("view-{k}.png"));
                 let img = v.to_srgb8();
                 let png = lightcraft_codecs::encode_png(&lightcraft_codecs::EncodeImage::rgba8(&img), &lightcraft_codecs::EncodeMeta::default())