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/Cargo.toml

@@ -10,6 +10,7 @@ workspace = true
 
 [dependencies]
 fieldcraft-engine = { workspace = true }
+fieldcraft-lbm-gpu = { workspace = true }
 fieldcraft-gpu = { workspace = true }
 fieldcraft-render = { workspace = true }
 fieldcraft-io = { workspace = true }

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") => {

apps/fieldcraft/Cargo.toml

@@ -21,6 +21,7 @@ neg_cmp_op_on_partial_ord = "allow"
 
 [dependencies]
 fieldcraft-engine = { workspace = true }
+fieldcraft-lbm-gpu = { workspace = true }
 fieldcraft-ui-egui = { workspace = true }
 fieldcraft-io = { workspace = true }
 fieldcraft-update = { workspace = true }

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(()),