refactor: implement service-based strand with pooled impls

bench/bench.cpp

@@ -10,7 +10,7 @@
 #include <boost/capy/ex/async_run.hpp>
 #include <boost/capy/ex/execution_context.hpp>
 #include <boost/capy/ex/run_on.hpp>
-#include <boost/capy/strand.hpp>
+#include <boost/capy/ex/strand.hpp>
 #include <boost/capy/task.hpp>
 
 #include <chrono>

include/boost/capy.hpp

@@ -60,7 +60,7 @@
 #include <boost/capy/core/polystore_fwd.hpp>
 #include <boost/capy/ex/run_on.hpp>
 #include <boost/capy/core/small_unique_ptr.hpp>
-#include <boost/capy/strand.hpp>
+#include <boost/capy/ex/strand.hpp>
 #include <boost/capy/task.hpp>
 #include <boost/capy/core/thread_local_ptr.hpp>
 #include <boost/capy/ex/thread_pool.hpp>

include/boost/capy/ex/detail/strand_service.hpp

@@ -0,0 +1,78 @@
+//
+// Copyright (c) 2025 Vinnie Falco (vinnie dot falco at gmail dot com)
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+//
+// Official repository: https://github.com/cppalliance/capy
+//
+
+#ifndef BOOST_CAPY_EX_DETAIL_STRAND_SERVICE_HPP
+#define BOOST_CAPY_EX_DETAIL_STRAND_SERVICE_HPP
+
+#include <boost/capy/detail/config.hpp>
+#include <boost/capy/ex/execution_context.hpp>
+
+#include <cstddef>
+
+namespace boost {
+namespace capy {
+namespace detail {
+
+// Forward declaration - full definition in src/
+struct strand_impl;
+
+//----------------------------------------------------------
+
+/** Service that manages pooled strand implementations.
+
+    This service maintains a fixed pool of strand_impl objects.
+    When a strand is constructed, it obtains a pointer to one
+    of these pooled implementations based on a hash.
+
+    @par Thread Safety
+    The service operations are thread-safe.
+*/
+class BOOST_CAPY_DECL strand_service
+    : public execution_context::service
+{
+    class impl;
+    impl* impl_;
+
+public:
+    /** Construct the strand service.
+
+        @param ctx The owning execution context.
+    */
+    explicit
+    strand_service(execution_context& ctx);
+
+    /** Destructor.
+    */
+    ~strand_service();
+
+    /** Return a pointer to a pooled implementation.
+
+        Uses a hash to select an implementation from the pool.
+        The salt is incremented after each call to distribute
+        strands across the pool.
+
+        @return Pointer to a strand_impl from the pool.
+    */
+    strand_impl*
+    get_implementation();
+
+protected:
+    /** Shut down the service.
+
+        Called when the owning execution context shuts down.
+    */
+    void
+    shutdown() override;
+};
+
+} // namespace detail
+} // namespace capy
+} // namespace boost
+
+#endif