generate/constraint_comprehension/solver.py: solve_two_var_linear (order-independent 2x2 integer Cramer's rule over typed constraints), the solve_two_var_count_weight specialization, and solve/answer_constraint_problem driving it from a ConstraintProblem. Four typed refusals: indistinguishable_weights (det==0), non_integer_solution (numer%det!=0, never rounds), negative_solution, verification_failed (identity backstop).
Ties to the C2 gold: solves all 7 solved fixtures to their gold value and refuses all 3 solver_refuses fixtures with EXACTLY the gold-claimed reason (the gold's reason is now solver-verified, not just annotation). Per-refusal meaningful-fail + positive re-substitution. Off-serving. 9 tests.
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