Add mod-factor-propagation lemma to NLA divisions solver

When a monic x*y has a factor x with mod(x, p) = 0 (fixed), propagate
mod(x*y, p) = 0. This enables Z3 to prove divisibility properties like
x mod p = 0 => (x*y) mod p = 0, which previously timed out even for
p = 2. The lemma fires in the NLA divisions check and allows Gröbner
basis and LIA to subsequently derive distributivity of div over addition.

Extends division tuples from (q, x, y) to (q, x, y, r) to track the
mod lpvar. Also registers bounded divisions from the mod internalization
path in theory_lra, not just the idiv path.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/test/CMakeLists.txt

@@ -87,6 +87,7 @@ add_executable(test-z3
   memory.cpp
   model2expr.cpp
   model_based_opt.cpp
+  mod_factor.cpp
   model_evaluator.cpp
   model_retrieval.cpp
   monomial_bounds.cpp