lbm-gpu: the lattice Boltzmann step on the GPU, checked against the CPU

A new L7 crate: the CPU scheme's collision (TRT with Guo forcing), the
per-link streaming rules (bounce-back with Ladd's term, slip reflection,
open placeholders), the Zou–He boundaries and the momentum exchange per
force group as wgpu compute shaders, stepping the flow fieldcraft_lbm's
Flow prepared, through its Backend trait: populations stay on the device,
a chunk of steps is one submission, and only frames, forces and the
stability check come back. Populations are stored as deviations from
their weights, which keeps single precision honest: the verification
cases (Poiseuille with Λ = 3/16, Couette with wall forces, Taylor–Green,
a sphere's drag, a Zou–He channel) agree with the CPU to 2–5e-6 in
velocity, 1e-6 in density and 5e-5 in force, with those tolerances stated
in the tests. The rules are applied one thread per link rather than in a
per-cell loop, which lavapipe's JIT miscompiles (and which diverges on
real GPUs anyway). Native only for now; the browser registers nothing.

Part of #529.

docs/architecture.md

@@ -33,6 +33,7 @@ crates in strictly lower layers.
 | L5 | `fieldcraft-io` | `.fieldcraft` files (docs/formats/fieldcraft.md): text model definition, binary mesh, geometry references with content hashes, the journal; atomic saves; autosave storage for crash recovery; read and write roots that confine an automation client's file access (`roots`). Bytes in, bytes out; the file system only in `fs`, `recovery` and `roots` (in the browser, files kept in memory: `memory`). Result files later. | started |
 | L6 | `fieldcraft-engine` | Commands (`CommandSpec`), `Session::execute` with rollback and the panic guard, scripts; every file a command names checked against the session's roots (`Session::file_access`); headless pictures of the session (`view`: model, geometry, deformed shape and contours on the CPU rasterizer). | started |
 | L7 | `fieldcraft-gpu` | The viewport's GPU pipelines (lit faces, wide lines, MSAA, depth) without a window toolkit, the adapter policy (`FIELDCRAFT_GPU`: hardware GPU, software GPU, CPU rasterizer) and offscreen pictures (`fieldcraft-cli snapshot`). The window draws through the same pipelines. | started |
+| L7 | `fieldcraft-lbm-gpu` | The lattice Boltzmann solver's GPU backend: the CPU scheme's step (TRT collision with Guo forcing, the per-link wall rules, Zou–He boundaries, momentum exchange) as wgpu compute shaders, driven through `lbm`'s `Backend` trait from the flow the CPU prepared; single precision, deterministic per device, checked against the CPU within its tests' tolerances; opt-in per solve (`solve.fluid` with `backend: "gpu"`). Opens its device with `gpu`'s adapter policy. | started |
 | L7 | `fieldcraft-ui-egui`, `fieldcraft-mcp` | Desktop UI in the Craft family's layout (title bar with quick actions, workspace ribbon from the catalogue, browser, viewport with navigation bar and orientation gizmo, context panel, command search and command line, journal timeline; native file pickers as host services, background opening and solving; results display with legend and probe; offscreen capture of the whole window) and control channel requests (docs/control-protocol.md); MCP server over stdio (docs/mcp.md): tools over engine commands, a headless session or the running app through its control channel, read and write roots. | ui, mcp started |
 | testkit | `fieldcraft-testkit` | Test tooling, a dev-dependency only: the meshing test corpus (`tests/corpus/*/recipe.json`, analytic shapes) and its runner (`cargo xtask corpus`). | started |
 | apps | `fieldcraft`, `fieldcraft-cli`, `fieldcraft-web` | Desktop, command line, browser (docs/web.md). | started |