Fix derivative instability and recursion bugs

- Add top-level cache (m_top_cache) to ensure stable AST node identity
  across repeated derivative calls, preventing state graph divergence
- Add get_head_tail helper for derive_to_re with str.is_unit/str.is_concat
- Add ITE hoisting in mk_union/mk_inter to keep ITEs at top level
- Add De Morgan rule in mk_complement: ~(A∪B) → ~A ∩ ~B
- Add ~ε → .+ simplification in mk_complement
- Add prefix factoring: a·x ∪ a·y = a·(x∪y) and a·x ∩ a·y = a·(x∩y)
- Add r* ∩ .+ = r+ special case in mk_inter
- Enhance is_subset with union/intersection distributivity and complement
- Remove De Morgan from mk_inter to prevent infinite recursion loop

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

src/ast/rewriter/seq_derive.h

@@ -56,6 +56,7 @@ namespace seq {
 
         // Cache: maps (ele, regex) pair to its derivative
         obj_pair_map<expr, expr, expr*> m_cache;
+        obj_pair_map<expr, expr, expr*> m_top_cache; // post-simplify cache
         expr_ref_vector      m_trail;    // pin cached results
 
         // Depth limiting
@@ -104,6 +105,9 @@ namespace seq {
         // Distribute concatenation through ITE/union in derivative
         expr_ref mk_deriv_concat(expr* d, expr* tail);
 
+        // Extract head character and tail from a sequence expression
+        bool get_head_tail(expr* s1, expr* s2, expr_ref& hd, expr_ref& tl);
+
         // Lightweight subsumption check: returns true if L(a) ⊆ L(b)
         bool is_subset(expr* a, expr* b);