Add OP_SEQ_POWER to seq_decl_plugin and create euf_snode/euf_sgraph for string graph encapsulation

Co-authored-by: NikolajBjorner <3085284+NikolajBjorner@users.noreply.github.com>

src/ast/euf/euf_sgraph.h

@@ -0,0 +1,95 @@
+/*++
+Copyright (c) 2025 Microsoft Corporation
+
+Module Name:
+
+    euf_sgraph.h
+
+Abstract:
+
+    Sequence/string graph layer
+
+    Encapsulates string expressions in the style of euf_egraph.h.
+    The sgraph registers sequence expressions and classifies them
+    into a ZIPT-style representation of string tokens organized
+    as binary concatenation trees.
+
+    This provides a layer that maps Z3 AST expressions (from seq_decl_plugin)
+    to snode structures with metadata for ground, regex-free, nullable, etc.
+
+Author:
+
+    Nikolaj Bjorner (nbjorner) 2025-03-01
+
+--*/
+
+#pragma once
+
+#include "util/region.h"
+#include "util/statistics.h"
+#include "ast/ast.h"
+#include "ast/seq_decl_plugin.h"
+#include "ast/euf/euf_snode.h"
+
+namespace euf {
+
+    class sgraph {
+
+        struct stats {
+            unsigned m_num_nodes;
+            unsigned m_num_concat;
+            unsigned m_num_power;
+            stats() { reset(); }
+            void reset() { memset(this, 0, sizeof(*this)); }
+        };
+
+        ast_manager&     m;
+        seq_util         m_seq;
+        region           m_region;
+        snode_vector     m_nodes;
+        expr_ref_vector  m_exprs;       // pin expressions
+        unsigned_vector  m_scopes;
+        unsigned         m_num_scopes = 0;
+        stats            m_stats;
+
+        // maps expression id to snode
+        ptr_vector<snode> m_expr2snode;
+
+        snode* mk_snode(expr* e, snode_kind k, unsigned num_args, snode* const* args);
+        snode_kind classify(expr* e) const;
+        void compute_metadata(snode* n);
+
+    public:
+        sgraph(ast_manager& m);
+        ~sgraph();
+
+        ast_manager& get_manager() const { return m; }
+        seq_util& get_seq_util() { return m_seq; }
+
+        // register an expression and return its snode
+        snode* mk(expr* e);
+
+        // lookup an already-registered expression
+        snode* find(expr* e) const;
+
+        // build compound snodes
+        snode* mk_empty(sort* s);
+        snode* mk_concat(snode* a, snode* b);
+        snode* mk_power(snode* base, snode* exp);
+
+        // scope management for backtracking
+        void push();
+        void pop(unsigned num_scopes);
+
+        // access
+        snode_vector const& nodes() const { return m_nodes; }
+        unsigned num_nodes() const { return m_nodes.size(); }
+
+        // display
+        std::ostream& display(std::ostream& out) const;
+
+        void collect_statistics(statistics& st) const;
+    };
+
+}
+