fix: [AI] deduplicate alias equations after substitution in find_perfect_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>

Author: AayushSabharwal

src/systems/alias_elimination.jl

@@ -197,6 +197,29 @@ function find_perfect_aliases!(
         end
     end
 
+    # After substitution, alias equations that connected a sticky non-target
+    # variable to a zero-priority variable become structurally identical to the
+    # direct alias between the sticky variable and the target (since the
+    # zero-priority variable was redirected to the target in the graph). Remove
+    # all but the first copy of each (v_a, v_b) variable pair.
+    let seen = Set{Tuple{Int,Int}}()
+        eqs_rm_set = Set(eqs_to_rm)
+        removed_additional_eqs = false
+        for (ieq, _, _) in candidate_eqs
+            ieq in eqs_rm_set && continue
+            snbors = 𝑠neighbors(graph, ieq)
+            length(snbors) == 2 || continue
+            pair = minmax(snbors[1], snbors[2])
+            if pair in seen
+                removed_additional_eqs = true
+                push!(eqs_to_rm, ieq)
+            else
+                push!(seen, pair)
+            end
+        end
+        removed_additional_eqs && sort!(eqs_to_rm)
+    end
+
     @set! sys.eqs = eqs
     state.sys = sys