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/test/CMakeLists.txt

@@ -132,6 +132,7 @@ add_executable(test-z3
   sls_test.cpp
   sls_seq_plugin.cpp
   seq_nielsen.cpp
+  nseq_basic.cpp
   small_object_allocator.cpp
   smt2print_parse.cpp
   smt_context.cpp