fix: [AI] fix bug caused by duplicate alias equations from eliminate_perfect_aliases!#4523
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When all variables in an alias group have equal state_priority (common for rotation matrix elements), findmax on equal values returned the first element in insertion order rather than the physics variable. For R[i,j] alias groups of ~18 variables, the physics variable (body₊frame_a₊R[i,j]) has vtd=Int and appears in ~9 equations, while visualization shape variables appear in only 1. Picking a shape variable as target and then eliminating the physics variable causes structural singularity in Pantelides. Fix: when priorities are tied, prefer the variable with the most equation appearances. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
…ect_aliases! When a sticky (high state_priority) non-target variable is directly aliased to a zero-priority variable in the same group, the c1/c2 removal check correctly keeps the equation (c1 ≠ c2). After the substitution loop rewrites that zero-priority variable to the group target, the surviving equation becomes structurally identical to the direct alias between the sticky variable and the target — a redundant duplicate. Multiple such duplicates over-constrain the bipartite graph and cause Pantelides to fail on downstream acceleration variables. Fix: after the substitution loop, scan surviving candidate equations for pairs with the same (v_a, v_b) variable endpoints and remove all but the first. This is O(n) in the number of candidate alias equations. The equation-count tiebreaker in pick_alias_target is retained as a secondary heuristic to prefer targets that already have derivative chains in var_to_diff, reducing unnecessary var_derivative! calls. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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The commit messages describe this pretty well. I'm trying to get Claude to write a test.