Restructure beman benchmark with bridges, statistics, and rigor

- Restructure beman benchmark into 3 tables x 3 stream types
- Add awaitable/sender bridge columns to benchmark
- Replace callback pipeline runners with repeat_effect_until
- Normalize coroutine function names
- Improve measurement precision and remove unused parameters
- Generalize bridge implementations and improve benchmark rigor
- Make awaitable_sender fully general: define get_io_executor
  query CPO, extract executor from P2300 scheduler via standard
  forwarding queries, remove all benchmark-specific types from
  bridge code
- Make sender_awaitable universally safe: atomic exchange protocol
  handles synchronous completion during start() without UB
- Add as_awaitable member to awaitable_sender so beman's
  await_transform bypasses sender_awaitable wrapping
- Make sender_thread_pool inherit execution_context, eliminating
  separate bench_context and null pointer in executor adapter
- Add multi-run statistics (5 runs, mean +/- stddev)
- Replace magic numbers with named constants
- Fix row labels to show abstraction level, not stream type
- Reorder tables: sender/receiver pipeline first

Author: sgerbino

bench/beman/allocation_tracker.hpp

@@ -0,0 +1,39 @@
+//
+// Copyright (c) 2026 Steve Gerbino
+//
+// 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_BENCH_ALLOCATION_TRACKER_HPP
+#define BOOST_CAPY_BENCH_ALLOCATION_TRACKER_HPP
+
+#include <atomic>
+#include <cstddef>
+#include <cstdlib>
+#include <new>
+
+static std::atomic<int64_t> g_alloc_count{0};
+
+void* operator new(std::size_t n)
+{
+    g_alloc_count.fetch_add(1, std::memory_order_relaxed);
+    void* p = std::malloc(n);
+    if (!p)
+        throw std::bad_alloc();
+    return p;
+}
+
+void operator delete(void* p) noexcept
+{
+    std::free(p);
+}
+
+void operator delete(void* p, std::size_t) noexcept
+{
+    std::free(p);
+}
+
+#endif