Add simplify_ite_rec and eval for two-phase derivative

- Add simplify_ite post-processing in operator() to simplify ITE conditions
- Add simplify_ite_rec(cond, sign, r) for propagating condition truth values
- Handles c == cond, x=ch1 vs x=ch2 with different constants
- Add eval(ele, d) for efficient two-phase: symbolic derivative + concrete eval
- mk_derivative uses two-phase pattern: m_derive(r) then m_derive.eval(ele, d)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/ast/rewriter/seq_rewriter.cpp

@@ -2918,7 +2918,9 @@ expr_ref seq_rewriter::mk_derivative(expr* r) {
 }
 
 expr_ref seq_rewriter::mk_derivative(expr* ele, expr* r) {
-    return m_derive(ele, r);
+    // Compute symbolic derivative (cached per regex), then evaluate for concrete element
+    expr_ref d = m_derive(r);
+    return m_derive.eval(ele, d);
 }