Rotated free-slip with every rheology and both momentum solvers: a test

[lmoresi]

Rotated strong free-slip is the free-slip condition we recommend, so it has to hold whatever the constitutive model. This adds the test that says it does.

A probe matrix over viscous, viscoplastic (constant and pressure-dependent yield), viscoelastic-plastic (BDF, ETD, with yield, the deformation step, the log-conformation history) and transversely isotropic VEP, in Stokes and the Eulerian NavierStokes, on a box and an annulus, converged at every step in serial and on 2 ranks — nothing needed fixing. tests/test_1018_rotated_freeslip_every_rheology.py (level_2, tier_b, 26 cases, ~2 min on 8 workers) records it:

• box: rms(v·Γ)/rms(v) < 1e-12 for every rheology and both solvers (measured 3e-17);
• annulus: the constraint holds at the nodes along their nodal normals, so v·Γ against the facet normal is the polygonal boundary's geometric error — first order in h (8.2e-2, 4.6e-2, 2.4e-2 at h = 1/4, 1/8, 1/16) and the same for every model. Each model is held to the viscous value on the same mesh (within 25 %).

Not covered here: preconditioner_flux="model" is refused on the rotated path (on feature/ve-stretch-split, which lands after #800); the adjoint of a rotated solve is #846.

Checklist (UW3 Style Charter — docs/developer/UW3_STYLE_CHARTER.md)

• I have read the Style Charter and followed it over the surrounding code (S2)
• No drive-by refactors, renames, or "while I was here" cleanups (S9)
• Bug fixes ship the regression test, written first, with level_* and tier_* markers (S8)
• No hedging names (maybe_ / try_ / do_) and no commented-out code (S3, S4)
• Every exception swallow states its sanctioned failure mode in a comment (S4)
• New data access uses .array / mesh.X.coords — no with ....access(...), no mesh.data (S7)
• Parallel safety considered — np2/np4 checked where swarm/mesh/solver dispatch is touched (S11)
• No pixi.toml / pixi.lock or other dependency changes riding in an unrelated PR
• AI-assisted work carries the attribution line below

Underworld development team with AI support from Claude Code

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