Fix fd gather priority - #3016
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`highest_priority` ranked operands by `_fd_priority`, which an `Add` or `Mul` reports as the generic `Differentiable` fallback rather than as the priority of the Functions it is built from. Operands carrying quite different staggering therefore tied there, and `sorted` being stable the winner fell out of SymPy's argument ordering -- which differs between an `EvalDerivative` and the `IndexDerivative` standing for the same expression, so `expand` silently decided where a product was gathered. On an elastic TTI gradient that put the `I * C * I^T` contraction on the strain instead of on the node-centred stiffness, leaving an outer interpolation that the NODE block would not have needed. With `expand=False` the second-order error then fell with slope 1.46 rather than 2, and scaling a kernel by two changed the result by 1.27 rather than by 2.
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Fails without the fix: `highest_priority` comes back with the staggered Function, the two sums both reporting the `Differentiable` fallback (.5) rather than the NODE Function's 1.2, and so tying.
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Codecov Report✅ All modified and coverable lines are covered by tests. Additional details and impacted files@@ Coverage Diff @@
## main #3016 +/- ##
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Coverage 83.72% 83.72%
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Files 257 257
Lines 54822 54838 +16
Branches 4693 4694 +1
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+ Hits 45901 45915 +14
- Misses 8110 8111 +1
- Partials 811 812 +1
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