Dissipative terms (M4)

Milestone M4 (#65) adds the dissipative terms to FLUME: viscosity and heat conduction (Navier–Stokes) on Euler and MHD, resistivity (Ohmic diffusion) on MHD, and the no-slip walls. Each coefficient is read either as a coefficient or as its dimensionless number (#49 N4); the keys are listed in input, the fluxes in numerics, the walls in boundary conditions.

The legs grow from the contract (DC) through exact linear and steady solutions on uniform grids (VV-1 to VV-6) to the 2:1 AMR and forest seams (VV-7, VV-8), and end with the run-by-run comparison of the two backends. Every leg runs on the CPU and on FNL, on 2 ranks.

Case	ID	What it verifies
Input contract	DC	25 input cases: invalid combinations refused with a message, implied coefficients logged exactly, zero coefficients bitwise inert, dimensional keys converted back exactly
Viscous waves	VV-1, VV-2	order 4 and 2 of the viscous flux on a shear wave (along $x$, $y$ and the diagonal), $\mu$ and $k$ together on an acoustic wave, against the exact linear mode
Compressible Couette flow	VV-3	the no-slip walls (isothermal and moving) and the viscous heating, against the exact steady profile
Becker's viscous shock	VV-4	the nonlinear viscous and heat fluxes through a resolved shock, Mach 2 and 3
Ohmic decay and the visco-resistive Alfvén wave	VV-5	the Ohmic flux (sign, cross terms, coefficient), $\mu$ and $\eta$ together, the viscous fluxes on MHD
Ohmic heating and the divergence constraint	VV-6	the Ohmic energy returned to the gas as heat; no divergence added by the curl form (GLM, EGLM)
Conservation across 2:1 seams and forest seams	VV-7	the dissipative fluxes carried by the reflux and the inter-realm register; forest union equal to the single realm
Accuracy across 2:1 seams	VV-8	the order of the composite grid (second), and the reflux as the carrier of the dissipative seam flux
CPU and FNL agree run by run	VV agree	every one of the 72 VV runs equal on the two backends within ${10}^{-10}$ of the physical scales

Run every leg:

cd src/tests/flume/verification/dissipation && ./check.sh                   # DC, CPU
cd src/tests/flume/verification/viscous && ./check.sh                       # VV-1 to VV-8, CPU
FLUME_EXE=$PWD/../../../../../exe/adam_flume_fnl ./check.sh                 # FNL
./check.sh --legs agree                                                     # CPU against FNL

The figures (viscous.png, viscous-fields.png, couette.png) are drawn by make_doc_figures.py from the CPU work directories that viscous/check.sh leaves (no make_doc_runs.sh group):

exe/.regression-venv/bin/python src/tests/flume/verification/make_doc_figures.py --out docs/public/flume \
   --only viscous viscous-fields couette