Issue #168: the desktop app and CLI log warnings to stderr; RUST_LOG works

lightcraft-cli installed no `log` backend, so every log::warn!/info! record
(the GPU-to-CPU fallback, an unknown LIGHTCRAFT_GPU_BACKEND, the adapter
chosen) was dropped and RUST_LOG, which docs/gpu-pipeline.md documents for
the fallback warning, did nothing. The desktop app had its own logger that
read only LIGHTCRAFT_LOG.

Both now install lightcraft_engine::logging: warnings and errors on stderr
by default; LIGHTCRAFT_LOG or RUST_LOG (off, error, warn, info, debug,
trace) picks another level, LIGHTCRAFT_LOG winning, and RUST_LOG's
per-target directives (`lightcraft=debug,wgpu=warn`) set LightCraft's
level. Below warnings only LightCraft's own records are shown, as the app's
logger did. The app's --help lists the variable.

Tests: level parsing and target filtering (engine logging::tests); the CLI
prints "LIGHTCRAFT_GPU_BACKEND=bogus names no known backend" with
RUST_LOG=warn and nothing with RUST_LOG=off (cli.rs).

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01K3vVft4VXvvy8SpbbUceTW

apps/lightcraft-cli/src/main.rs

@@ -122,6 +122,9 @@ fn main() -> ExitCode {
         dhat::Profiler::builder().file_name(file).build()
     };
     alloc_release::install();
+    // Warnings (a GPU render redone on the CPU, an unknown backend name) on stderr; LIGHTCRAFT_LOG
+    // or RUST_LOG picks another level (#168).
+    lightcraft_engine::logging::install("lightcraft-cli");
     let args: Vec<String> = std::env::args().skip(1).collect();
     let r = match args.first().map(String::as_str) {
         Some("run") => run(&args[1..]),

apps/lightcraft-cli/tests/cli.rs

@@ -336,3 +336,29 @@ fn connect_failure_hint_names_the_attempted_port() {
     assert!(err.contains("--control 18437"), "the hint must name the port it tried: {err}");
     assert!(!err.contains("--control 7980"), "{err}");
 }
+
+/// The CLI logs warnings on stderr (#168): with `RUST_LOG=warn` an unknown GPU backend name is
+/// reported, and `RUST_LOG=off` silences it. Before, no logger was installed and nothing appeared.
+#[test]
+fn warnings_are_logged_on_stderr() {
+    let input = tmp("log-in.png");
+    gradient_png(&input);
+    let out = tmp("log-out.jpg");
+    let run = |level: &str| {
+        let o = Command::new(BIN)
+            .args(["render", input.to_str().unwrap(), "-o", out.to_str().unwrap()])
+            .env("LIGHTCRAFT_GPU_BACKEND", "bogus")
+            .env("RUST_LOG", level)
+            .env_remove("LIGHTCRAFT_LOG")
+            .env_remove("LIGHTCRAFT_GPU")
+            .output()
+            .unwrap();
+        let stderr = String::from_utf8_lossy(&o.stderr).into_owned();
+        assert!(o.status.success(), "{stderr}");
+        stderr
+    };
+    let warn = run("warn");
+    assert!(warn.contains("LIGHTCRAFT_GPU_BACKEND=bogus names no known backend"), "{warn}");
+    let off = run("off");
+    assert!(!off.contains("names no known backend"), "{off}");
+}