feat(exactTriangle): prove modular counts and selected-prime bounds - #1445

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Not declared at the PR head: `TriangleInstance`, `AlmanWilliams2026`, `CyclicRing`. Advisory only; see the job summary.

`Cslib.Algorithms.ExactTriangle.matP` and `Cslib.CyclicIntRing.coeff` (Complexity/Foundations/RingTheory/CyclicIntRing.lean:54) share a 211-node subterm, 47% of the smaller body, built from `Cslib.CyclicIntRing`, `Polynomial`, `Polynomial.semiring`, `Polynomial.X`, `RingHom`, `Polynomial.C`. Advisory only; see the job summary.

`Cslib.Algorithms.ExactTriangle.matQ` and `Cslib.CyclicIntRing.coeff` (Complexity/Foundations/RingTheory/CyclicIntRing.lean:54) share a 211-node subterm, 47% of the smaller body, built from `Cslib.CyclicIntRing`, `Polynomial`, `Polynomial.semiring`, `Polynomial.X`, `RingHom`, `Polynomial.C`. Advisory only; see the job summary.