Quick-fix batch: pressure nullspace, mesh cache filename, test number

[lmoresi]

Closes #577. Closes #600. Closes #618.

Three small independent fixes, batched because none warrants its own change —
the same shape as #456 and #532.

#577 — the zero-slip annulus ran a singular saddle

CylindricalStokes.apply_boundary_conditions returned before the line that
removes the pressure nullspace:

if self.boundary == "zero":
    prescribed_velocity(solver, self.boundaries, (0.0, 0.0))
    return                                   # <- before the nullspace line

free_slip(solver, self.boundaries)
solver.petsc_use_pressure_nullspace = True

Both cases are an enclosed annulus, so in both the pressure is determined only
up to a constant. The nullspace belongs to the domain, not to the wall type, and
is now set once after the branch.

Measured, reading the flag back off the solver:

boundary	before	after
"zero"	unset	True
"free"	True	True

#618 — one cache file for every resolution

uw_filename = f"{mesh_file_dir()}/uw_structuredQuadBox_minC{minCoords}_maxC{maxCoords}.msh"

elementRes was not in the name, so every StructuredQuadBox on the same box
wrote and read one path whatever its resolution, and two processes at different
resolutions raced on it. The annulus already carries its cell size for this
reason. Resolution is now part of the name; a 4x4 and an 8x8 produce
..._res(4, 4).msh and ..._res(8, 8).msh where they previously produced one
file between them.

#600 — two test files numbered 1029

test_1029_analytic_zhong2008.py collided with
test_1029_analytic_faulted_medium.py; the two PRs were written against
different bases and neither could see the other's choice. Zhong moves to 1030,
the faulted medium keeps 1029 because it merged first. The reference in
docs/developer/subsystems/analytic-solutions.md is updated and no stale
mention remains anywhere in src/, tests/, docs/ or scripts/.

Verified

Full ./uw test: 1556 passed, 32 skipped, 2 xfailed. The analytic contract,
conformance and Zhong files: 132 passed.

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