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.

Cargo.lock

@@ -1271,6 +1271,7 @@ dependencies = [
  "embed-resource",
  "fieldcraft-engine",
  "fieldcraft-io",
+ "fieldcraft-lbm-gpu",
  "fieldcraft-ui-egui",
  "fieldcraft-update",
  "getrandom 0.4.3",
@@ -1290,6 +1291,7 @@ dependencies = [
  "fieldcraft-engine",
  "fieldcraft-gpu",
  "fieldcraft-io",
+ "fieldcraft-lbm-gpu",
  "fieldcraft-mcp",
  "fieldcraft-render",
  "fieldcraft-update",