solve.fluid: backend "cpu" (the default) or "gpu"

The CPU stays the default, so every existing result and fixture is
bit-identical; "gpu" steps on the device FIELDCRAFT_GPU picks through the
registered backend, and is an error when the build has none. The desktop
app and the CLI (hence the MCP server) register fieldcraft-lbm-gpu at
start; the summary names what stepped. Hostile values are errors.

Part of #529.

apps/fieldcraft/src/main.rs

@@ -224,6 +224,8 @@ fn parse_args(args: impl IntoIterator<Item = String>, env_port: Option<String>)
 }
 
 fn main() -> eframe::Result {
+    // The lattice Boltzmann GPU backend, for `solve.fluid` with `backend: "gpu"`.
+    fieldcraft_lbm_gpu::register();
     let args = match parse_args(std::env::args().skip(1), std::env::var("FIELDCRAFT_CONTROL_PORT").ok()) {
         Ok(Some(a)) => a,
         Ok(None) => return Ok(()),