Add initial async::context test infrastructure

[kammce]

This PR introduces basic testing infrastructure for the async context system, demonstrating coroutine scheduling and stack memory management.

Changes

• Added coroutine test case: Implements coro_print() coroutine that exercises co_await with time-based suspension
• Implemented test scheduler: Created my_scheduler that tracks sleep events and demonstrates the do_schedule() interface
• Stack memory setup: Tests the current strong_ptr<std::span<byte>> pattern for coroutine stack allocation
• Added TODO comments:
  • Issue Choose a better type/strategy for "stack memory" #18: Replace strong_ptr<std::span<byte>> with a safer, more ergonomic API
  • Issue Scheduler isn't being called #19: Fix scheduler invocation (currently not being called during co_await)

Current Limitations

The test reveals that the scheduler's do_schedule() is not being invoked during co_await suspension points. This needs investigation and is tracked in #19.

The current stack memory API requires two separate allocations (array + span) with no lifetime bond between them, identified as a safety concern in #18.

Test Coverage

• ✅ Coroutine creation and execution
• ✅ Multiple suspension points with co_await
• ✅ sync_wait() completing the coroutine
• ⚠️ Scheduler integration (not yet working)

PR message co-authored-by: Claude noreply@anthropic.com

Edit, the latest commit resolves #19

[kammce]

This change resolves #19