Aero QA Stretch Solvers

Stretch solver QA (TACS, SU2, Gmsh/meshio)

Read docs/guidebook/05-tacs.md, 06-gmsh-meshio.md, 07-su2.md; execute their "Check your work" lists.

Non-negotiables for this repo:

• Stretch results are calibration data, never design evidence. They cannot pass or fail a prototype; they can only flag model disagreement.
• TACS: check analysis.backend == "tacs" (else the analytic fallback ran); KS failure < 1 means margin; hand-check shell mass = area × t × rho. Runs inside micromamba env tacs (MAMBA_ROOT_PREFIX=tools/mamba).
• SU2: ok means ran+parsed; converged (rms density drop) is the separate verdict — never conflate them. CFD Mach is floored at 0.30 (compressible Euler is stiff below); the JSON records requested vs CFD Mach. Prebuilt 8.5 accepts only MESH_FORMAT= SU2|CGNS; MUSCL_FLOW= NO with JST.
• Gmsh meshes must carry physical groups for every boundary; verify marker element counts cover the boundary before blaming a solver.