[bgregoir]

Adds polynomial evaluation, the factor theorem, divisibility and root counting to theories/algebra/Poly.ec, following mathcomp (algebra/poly.v) with peval for horner and deg (degree + 1) for size.

PolyComRing (coefficients in a commutative ring)

• evaluation lemmas: pevalC, peval0, peval1, pevalX, pevalD, pevalN, pevalB, pevalZ, peval_sum, pevalMX, pevalMXn, pevalXn, pevalM, peval_exp, pevalXsubC;
• remainder theorem peval_factor;
• roots: root0, rootC, rootX, rootN, root_XsubC, factor_theorem;
• divisibility of polynomials, as a clone of Ideal.IdealComRing, and root_factor_theorem : root p a <=> (X - polyC a) %| p.

Poly (coefficients in an integral domain)

• rootM, rootZ, factor_theorem_deg;
• roots_finite: the roots of a nonzero polynomial form a list of size < deg p;
• max_poly_roots : p <> poly0 => all (root p) rs => uniq rs => size rs < deg p;
• the probability that a nonzero polynomial vanishes at a sampled point: mu_root_le (bound (deg p - 1) * max mu1) and mu_root_uni (uniform distribution).

PolyReduce now reuses the divisibility clone inherited from PolyComRing instead of its own copy, so its names move from PolyReduce.PIdeal.* to PolyReduce.BasePoly.PIdeal.*; nothing else in the repository refers to them.

No new axioms. The motivation is the Poly1305 almost-Δ-universal bound, assumed as an axiom in examples/ChaChaPoly.