Issue #136: Windows: use DX12 by default, honour WGPU_BACKEND, never let GPU init block startup

wgpu loads the driver of every backend in an instance's set while it
enumerates adapters. The compute device (crates/gpu) hard-coded
Backends::PRIMARY (Vulkan + DX12 + Metal) and ignored WGPU_BACKEND, and
was created from LightcraftApp::new on a background thread at launch, so
a crashing Vulkan driver (Intel UHD 630, igvk64.dll) killed the process
before the window appeared; the window (egui-wgpu) also listed Vulkan.

- lightcraft_gpu::backend: DX12 only on Windows (Metal on macOS, as before
  on Linux) for both the compute device and the window;
  LIGHTCRAFT_GPU_BACKEND=dx12|vulkan|metal|gl|auto|off, else WGPU_BACKEND;
  backends missing from the build fall back to the default.
- The device is created only when GPU rendering is enabled, after the
  first frame; the preference is applied before the window opens.
- Crash sentinel: gpu-init.marker in the settings folder around device
  creation; if left behind, the next launch starts with GPU rendering off
  and a notice saying how to turn it back on.
- Tests: backend parsing/selection, sentinel, window defaults, headless
  snapshot with LIGHTCRAFT_GPU=0 / LIGHTCRAFT_GPU_BACKEND=off.
- Docs: gpu-pipeline.md (backends, env vars, troubleshooting), whats-new,
  parity LR-BEHAV-GPU.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

apps/lightcraft-cli/tests/cli.rs

@@ -183,6 +183,42 @@ fn snapshot_subcommand_renders_the_ui_headlessly() {
     assert_eq!((d.width, d.height), (640, 480));
 }
 
+/// Issue #136: with the GPU switched off from the environment the UI starts and renders on the CPU,
+/// without creating a GPU device (no driver is loaded), and says why.
+#[test]
+fn snapshot_starts_without_a_gpu() {
+    let script = tmp("nogpu.jsonl");
+    std::fs::write(
+        &script,
+        format!(
+            "{}\n{}\n{}\n",
+            json!({"method": "ui.set", "params": {"view": "photoGrid"}}),
+            json!({"method": "engine.execute", "params": {"command": "app.gpu"}}),
+            json!({"method": "ui.screenshot"}),
+        ),
+    )
+    .unwrap();
+    for (var, value, reason) in [("LIGHTCRAFT_GPU", "0", "LIGHTCRAFT_GPU=0"), ("LIGHTCRAFT_GPU_BACKEND", "off", "LIGHTCRAFT_GPU_BACKEND=off")] {
+        let out = tmp(&format!("nogpu-{var}.png"));
+        let o = Command::new(BIN)
+            .env_remove("LIGHTCRAFT_GPU")
+            .env_remove("LIGHTCRAFT_GPU_BACKEND")
+            .env(var, value)
+            .args(["snapshot", "--demo", "--script", script.to_str().unwrap(), "-o", out.to_str().unwrap(), "--size", "480x320"])
+            .output()
+            .unwrap();
+        assert!(o.status.success(), "{var}: {}", String::from_utf8_lossy(&o.stderr));
+        let replies: Vec<Value> = String::from_utf8_lossy(&o.stdout).lines().map(|l| serde_json::from_str(l).unwrap()).collect();
+        assert!(replies.iter().all(|r| r["ok"] == true), "{replies:?}");
+        let gpu = &replies[1]["result"];
+        assert_eq!(gpu["available"], false, "{var}: {gpu}");
+        assert_eq!(gpu["adapter"], Value::Null, "{var}: no device was created: {gpu}");
+        assert!(gpu["reason"].as_str().is_some_and(|r| r.contains(reason)), "{var}: {gpu}");
+        let d = lightcraft_codecs::decode(&std::fs::read(&out).unwrap(), Default::default()).unwrap();
+        assert_eq!((d.width, d.height), (480, 320));
+    }
+}
+
 fn run_cli(args: &[&str], stdin: Option<&str>) -> (bool, Vec<Value>, String) {
     let mut child = Command::new(BIN).arg("run").args(args).stdin(Stdio::piped()).stdout(Stdio::piped()).stderr(Stdio::piped()).spawn().unwrap();
     if let Some(text) = stdin {