Move nseq_regex/state into smt/seq and seq_model into smt/; rename to seq_* prefix (#8984)

* Initial plan

* build verified: nseq_regex moved to smt/seq

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

* rename nseq_regex/state/model to seq_regex/state/model in smt/seq; add Clemens Eisenhofer as co-author

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

* seq_model: remove theory_nseq dependency; get family_id from seq_util

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

* Add comments for regex enhancements in seq_model

Added comments regarding future improvements for regex handling.

* Add comments for large exponent handling

Add comments for handling large exponents in seq_model.cpp

* Revise comments for clarity on sort usage

Updated comments to reflect review suggestions regarding the use of the sort of 'n'.

* seq_state: remove sgraph dep; seq_model: use snode sort for is_empty; remove NSB review comments

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

* Update seq_state.h

* Remove unnecessary include for smt_context.h

* move seq_model from smt/seq/ to smt/; fix seq_state.h add_str_mem typo

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

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: NikolajBjorner <3085284+NikolajBjorner@users.noreply.github.com>
Co-authored-by: Nikolaj Bjorner <nbjorner@microsoft.com>

src/smt/nseq_state.cpp

@@ -1,17 +0,0 @@
-/*++
-Copyright (c) 2026 Microsoft Corporation
-
-Module Name:
-
-    nseq_state.cpp
-
-Abstract:
-
-    Implementation of nseq_state.
-
-Author:
-
-    Nikolaj Bjorner (nbjorner) 2026-03-01
-
---*/
-#include "smt/nseq_state.h"