Pin UWexpression _op_priority back to sympy default (avoid Matrix/UWexpression regression)

The previous commit set MathematicalMixin._op_priority = 11.5 to win
sympy dispatch over Matrix (10.01) for the bare-variable composition
case. UWexpression inherits from MathematicalMixin (first in MRO), so
that change inadvertently bumped UWexpression's priority too — and
UWexpression has its OWN __rtruediv__ (and __rmul__, etc.) for sympy
interop. Its __rmul__ explicitly handles MutableDenseMatrix; its
__rtruediv__ does not, falling through to Symbol.__rtruediv__ which
fails on MatrixBase.

Result: solvers that compute Matrix / UWexpression (e.g. SNES_Diffusion's
F0 expression `self.DuDt.bdf(0) / self.delta_t` at solvers.py:2499)
broke with TypeError on every transient/Darcy/AdvDiff path. CI surfaced
this on test_1005, test_1006, test_1100, test_1110.

Fix: explicitly set UWexpression._op_priority = 10.0 to opt out of the
mixin's bump. UWexpression already handles Matrix dispatch correctly
through its own dunders (and via sympy's standard machinery for
fall-through cases); it doesn't need the priority bump that pure
MathematicalMixin classes (EnhancedMeshVariable, SwarmVariable) need.

Verified:
  - #137 4-case reproducer still passes (the original fix is intact).
  - Previously-failing tests all pass: test_1005_TransientDarcyCartesian,
    test_1006_RichardsCartesian, test_1100_AdvDiffCartesian,
    test_1110_advDiffAnnulus — 8/8 pass.
  - pytest -m "level_1 and tier_a" on amr-dev: 56 passed, 3 skipped,
    0 failed.

Underworld development team with AI support from Claude Code

Author: lmoresi

src/underworld3/function/expressions.py

@@ -629,6 +629,18 @@ class UWexpression(MathematicalMixin, uw_object, Symbol):
     # Slot for unique ID used in _hashable_content (like sympy.Dummy)
     __slots__ = ('_uw_id',)
 
+    # Override the MathematicalMixin priority bump back to sympy's default.
+    # MathematicalMixin sets _op_priority = 11.5 to win dispatch over
+    # sympy.Matrix (10.01) for the bare-variable composition case (#137 —
+    # MeshVariable / SwarmVariable on the right of a sympified subexpression).
+    # UWexpression is itself a sympy.Symbol subclass with its own __rmul__ /
+    # __rtruediv__ that already handle the Matrix case; inheriting the high
+    # priority would route Matrix / UWexpression through UWexpression's
+    # __rtruediv__ (which falls back to Symbol.__rtruediv__ → fails on
+    # MutableDenseMatrix). Pin it back to 10.0 so sympy's standard
+    # Matrix-dispatch path keeps handling these.
+    _op_priority = 10.0
+
     def __new__(
         cls,
         name,