feat(constraint): R2 linear-constraint IR — typed problem model (R2 C1)
generate/constraint_comprehension/{expr,model}.py: frozen, slots'd dataclasses, no behavior. expr = LinearExpr (sum(coeff*symbol)+constant) + LinearConstraint (lhs eq rhs, optional source_span). model = Unknown (symbol/entity/unit/finite-integer domain), AttributeFact (per-category coefficient provenance), ConstraintQuery (symbol+unit), ConstraintProblem (unknowns/facts/constraints/query).
Terms pinned as (symbol, coefficient) to match the gold serialization. Query is a minimal dedicated type, not R1's BoundUnknown (no degenerate fit). Off-serving package; no generate.derivation / reliability_gate import. 9 IR tests (shape + frozen + defaults).
e71531c0c9