lbm-gpu: the temperature lattice on the GPU

The D3Q7 scalar lattice steps on the device after the flow's step, in
its own A–B pair of buffers: BGK collision with the velocity the
momentum collision leaves in the moments buffer, pull streaming with
the links' rules read through each group's thermal condition
(anti-bounce-back for a held temperature, bounce-back for an adiabatic
face, a flux added to the bounced population), inlets at their held
temperature and outlets copying their neighbour. The populations are
stored as deviations from w_q T₀ so single precision keeps the digits
of the differences. The heat each link exchanged goes into the fourth
component of the momentum-exchange contributions and the group sums;
the buoyant acceleration −β g θ is read per cell in the collision, the
open boundaries and the moments. A temperature that stops being a
number ends the chunk with Error::Unstable like the flow's checks.

Flow exposes what the backend needs to build the device state (the
thermal settings and populations).

Verification against the CPU on lavapipe: plates held at 20 and 21
(temperatures within 1.4e-5 of the range, the wall heats within 6e-5
and both against k ΔT A / H), a heated channel with a parabolic inlet
carrying its temperature, a flux wall, a held wall and an outlet
(temperature 3e-7, heat 1e-7) and a buoyant cavity at Ra 10³
(velocity 1.3e-6, temperature 6e-7, heat 2.4e-6). The tests now fail
on a shader that does not compile instead of skipping; only a missing
device skips.

Part of #157.

crates/lbm-gpu/src/lib.rs

@@ -1,7 +1,7 @@
 //! The lattice Boltzmann solver on the GPU: [`GpuFlow`] steps a flow that
 //! [`fieldcraft_lbm::Flow`] prepared (grid, links, open cells, initial state) with the compute
 //! shaders of `lbm.wgsl`, which are the CPU scheme's collision, streaming rules, open boundaries
-//! and momentum exchange written for the device; `fieldcraft_lbm::run` drives it through the
+//! and momentum exchange, and with heat its temperature lattice, written for the device; `fieldcraft_lbm::run` drives it through the
 //! [`Backend`] trait in chunks of steps, so populations stay on the device and only frames,
 //! force histories and the stability check come back.
 //!