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-cli/src/main.rs

@@ -63,6 +63,8 @@ usage:
 const MAX_SCRIPT_BYTES: u64 = 256 * 1024 * 1024;
 
 fn main() -> ExitCode {
+    // The lattice Boltzmann GPU backend, for `solve.fluid` with `backend: "gpu"` (scripts and MCP).
+    fieldcraft_lbm_gpu::register();
     let args: Vec<String> = std::env::args().skip(1).collect();
     let result = match args.first().map(String::as_str) {
         Some("commands") if args.get(1).map(String::as_str) == Some("--json") => {