✨ Add proxy capability to context & add cancellation support

[kammce]

This PR adds a proxy system to the async context, introduces an unsafe cancellation routine for contexts, refactors a number of coroutine helpers and updates the test suite accordingly. The changes also include new benchmarks, build system tweaks, and minor clean‑ups.

Proxy provides a means for a coroutine to use its context to supervise other async operations without losing control over being the active coroutine.

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Key Functional Changes

Feature	What changed
Proxy contexts	context::borrow_proxy() now returns a proxy context that shares the same stack space as its parent but has its own active coroutine handle. The proxy owns a proxy_info struct that tracks the original “origin” and the parent. The proxy is created via a private constructor context(proxy_tag, context&).
Proxy state	The original m_proxy member was a scheduler_t; it is now a proxy_state variant (`proxy_info
Unsafe cancellation	context::unsafe_cancel() walks the coroutine chain starting from m_active_handle and destroys every coroutine up to the top‑level one. The stack is reset but the public future object that was bound to this context is not marked cancelled – hence “unsafe”.
Memory accounting	memory_used(), capacity(), and memory_remaining() were updated to use the new layout.
State handling	The context’s state() method now returns a blocked_by (defaulting to nothing) and the transition logic uses the proxy’s scheduler.
Future refactor	future now stores its state in a variant future_state<T> (`handle
Cancellation handling	future::cancel() destroys the coroutine and marks its state as cancelled_state. Helper operation_cancelled exception is thrown when awaiting a cancelled future.
Future construction	promise::get_return_object() now creates the future from the promise’s coroutine handle and records the frame size.
Future for void	future<void> has its own awaiter that throws if the future was cancelled or had an exception.
Context constructor	context(scheduler_t const&, usize) now initializes the proxy field. The old m_scheduler member is removed; scheduler is stored in proxy_state.
Proxy creation guard	borrow_proxy() is deleted for temporaries (context&&) to avoid dangling references.

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Test Additions

• Cancellation test – verifies that unsafe_cancel() correctly destroys all coroutines and resets the stack.
• Exception propagation test – checks that a thrown exception inside a coroutine propagates to the caller.
• Proxy coroutine tests – exercise proxies that run coroutines in a shared stack and handle timeouts.
• Memory accounting assertions – confirm that memory usage is zero after cancellation.

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Benchmark Additions

• Added a new benchmarks directory with the original benchmark file (benchmark.cpp) moved from benchmark.
• Updated CMake to link against Google Benchmark.
• Benchmarks cover:
  • Virtual calls
  • Futures that return synchronously or via coroutines
  • Mixed coroutine/sync patterns
  • Void‑return coroutines
  • Variant return type virtual calls

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Build System & Packaging

• CMakeLists.txt now:
  • Adds benchmarks/* to the source list.
  • Builds a separate async_benchmark executable only when not cross‑compiling.
• conanfile.py
  • Adds benchmarks to the exported sources.
  • Only adds test dependencies (boost-ext-ut, benchmark) when building for non‑baremetal OSes.

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