PostingPromise: Add dynamic post-to io_context targeting

This allows us to dynamically choose the target that a PostingPromise
coro will be posted to at runtime rather than only posting to the
statically configured ThreadTag::io_context() target. Big usability
improvement.

include/spinscale/co/postTarget.h

@@ -0,0 +1,29 @@
+#ifndef POST_TARGET_H
+#define POST_TARGET_H
+
+#include <type_traits>
+
+#include <boost/asio/io_context.hpp>
+
+namespace sscl::co {
+
+/**	Opt-in dynamic post-TO target for TaggedPostingPromise coroutines.
+ *	When omitted, initial_suspend posts to ThreadTag::io_context().
+ *	Post-back still uses callerIoContext (getSelf() at co_await site).
+ */
+struct ExplicitPostTarget
+{
+	boost::asio::io_context& ioContext;
+
+	explicit ExplicitPostTarget(boost::asio::io_context& ctx) noexcept
+	: ioContext(ctx)
+	{}
+};
+
+template<typename T>
+inline constexpr bool is_explicit_post_target_v =
+	std::same_as<std::remove_cvref_t<T>, ExplicitPostTarget>;
+
+} // namespace sscl::co
+
+#endif // POST_TARGET_H