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.

Cargo.lock

@@ -1401,6 +1401,19 @@ dependencies = [
  "thiserror 2.0.21",
 ]
 
+[[package]]
+name = "fieldcraft-lbm-gpu"
+version = "0.6.0"
+dependencies = [
+ "fieldcraft-gpu",
+ "fieldcraft-lbm",
+ "fieldcraft-mesh",
+ "fieldcraft-model",
+ "log",
+ "pollster 1.0.1",
+ "wgpu",
+]
+
 [[package]]
 name = "fieldcraft-mcp"
 version = "0.6.0"