Implement ZIPT string solver skeleton (theory_nseq)

Add theory_nseq, a Nielsen-graph-based string solver plugin for Z3.

## New files
- src/smt/nseq_state.h/.cpp: constraint store bridging SMT context to
  Nielsen graph with manual push/pop backtracking
- src/smt/nseq_regex.h/.cpp: regex membership handling via Brzozowski
  derivatives (stub delegates to sgraph::brzozowski_deriv)
- src/smt/nseq_model.h/.cpp: model generation stub
- src/smt/theory_nseq.h/.cpp: main theory class implementing smt::theory
  with its own private egraph/sgraph, returns FC_GIVEUP as skeleton
- src/test/nseq_basic.cpp: unit tests covering instantiation, parameter
  validation, trivial-equality SAT, and node simplification

## Extensions to seq_nielsen.h/.cpp
- Add search_result enum and solve() iterative-deepening DFS entry point
- Add search_dfs() recursive DFS driver
- Add simplify_node(), generate_extensions(), collect_conflict_deps()
- Add nielsen_node::simplify_and_init(): trivial removal, empty
  propagation, prefix matching, symbol clash detection
- Add nielsen_node::is_satisfied(), is_subsumed_by()
- Implement Det, Const Nielsen, and Eq-split modifiers in
  generate_extensions()

## Integration
- smt_params.cpp: accept 'nseq' as valid string_solver value
- smt_params_helper.pyg: document 'nseq' option
- smt_setup.h/.cpp: add setup_nseq(), wire into setup_QF_S() and
  setup_seq_str()
- smt/CMakeLists.txt: add new sources and smt_seq dependency

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

src/smt/smt_setup.cpp

@@ -35,6 +35,7 @@ Revision History:
 #include "smt/theory_seq_empty.h"
 #include "smt/theory_seq.h"
 #include "smt/theory_char.h"
+#include "smt/theory_nseq.h"
 #include "smt/theory_special_relations.h"
 #include "smt/theory_sls.h"
 #include "smt/theory_pb.h"
@@ -571,7 +572,9 @@ namespace smt {
         else if (m_params.m_string_solver == "auto") {
             setup_unknown();
         }
- 
+        else if (m_params.m_string_solver == "nseq") {
+            setup_nseq();
+        }
         else if (m_params.m_string_solver == "empty") {
             setup_seq();
         }
@@ -579,7 +582,7 @@ namespace smt {
             // don't register any solver.
         }
         else {
-            throw default_exception("invalid parameter for smt.string_solver, valid options are 'seq', 'auto'");
+            throw default_exception("invalid parameter for smt.string_solver, valid options are 'seq', 'auto', 'nseq'");
         }
     }
 
@@ -754,11 +757,14 @@ namespace smt {
         else if (m_params.m_string_solver == "none") {
             // don't register any solver.
         }
+        else if (m_params.m_string_solver == "nseq") {
+            setup_nseq();
+        }
         else if (m_params.m_string_solver == "auto") {            
                 setup_seq();
         } 
         else {
-            throw default_exception("invalid parameter for smt.string_solver, valid options are 'seq', 'auto'");
+            throw default_exception("invalid parameter for smt.string_solver, valid options are 'seq', 'auto', 'nseq'");
         }
     }
 
@@ -785,6 +791,11 @@ namespace smt {
         m_context.register_plugin(alloc(smt::theory_char, m_context));        
     }
 
+    void setup::setup_nseq() {
+        m_context.register_plugin(alloc(smt::theory_nseq, m_context));
+        setup_char();
+    }
+
     void setup::setup_finite_set() {
         m_context.register_plugin(alloc(smt::theory_finite_set, m_context));
     }