refactor: add make_*/async_* co_await-able I/O API for files and networking

[Copilot]

Summary

Exposes a coroutine-friendly async I/O surface over the existing file and networking primitives, enabling co_await-based usage without blocking task threads.

Changes

New public headers

• include/net.hpp — canonical replacement for networking.hpp (which becomes a backward-compat #include shim). Adds:
  • virtual make_read(uint32_t) / make_write(const char*, uint32_t) to tcp_network_stream and tcp_network_blocking
  • non-virtual make_accept(bool) to tcp_network_server
  • #include "future.hpp" so callers get future_ptr<T> in scope
• include/coroutine/net.hpp — thin async_read, async_write, async_accept wrappers returning future_ptr<T> that can be directly co_awaited
• include/coroutine/file.hpp — async_read, async_read_at, async_read_fixed, async_read_fixed_at, async_write, async_write_at, async_append wrappers over file_handle

Modified files

• include/networking.hpp — reduced to #include "net.hpp" shim for backward compatibility
• include/files.hpp — added make_read, make_read_at, make_read_fixed, make_read_fixed_at, make_write, make_write_at, make_append declarations on file_handle
• src/files.cpp — thin delegating implementations of all file_handle::make_*
• src/networking.cpp — make_read/make_write implemented in all four concrete classes (Windows stream, Windows blocking, Linux stream, Linux blocking); tcp_network_server::make_accept wraps accept_blocking in a future

Usage example

#include <coroutine/net.hpp>
#include <coroutine/file.hpp>

fast_task::task_auto_start_coro<void> handle(fast_task::networking::tcp_network_blocking& conn) {
    auto data = co_await fast_task::async_read(conn, 1024);
    co_await fast_task::async_write(conn, data.data(), (uint32_t)data.size());
}

fast_task::task_auto_start_coro<void> server_loop(fast_task::networking::tcp_network_server& srv) {
    auto* client = co_await fast_task::async_accept(srv);
    // client is tcp_network_blocking*, caller owns it
}

fast_task::task_auto_start_coro<void> read_file(fast_task::files::file_handle& f) {
    auto bytes = co_await fast_task::async_read(f, 4096);
}