Keep the callable passed to create_task alive while the task runs

[otamachan]

create_task(callable) invoked a temporary lambda and returned its task, but the lambda was destroyed at the end of the caller's full-expression. A coroutine lambda reads its captures through the closure, so any capture used after the first suspension was dangling:

auto task = ctx.create_task([&ctx, &count]() -> Task<void> {
  co_await ctx.sleep(1s);
  count++;  // the closure holding &count is already gone
});

This is the pattern the guide and the examples use. Under ASan, TimerStreamTest.BasicNext reported stack-use-after-scope (mentioned in #56).

The callable is now moved into a wrapper coroutine's frame, which awaits the task it returns, so the captures live as long as the task.

SleepTest.CreateTaskKeepsTheLambdaAlive checks that the lambda's captures are still alive after create_task returns. It fails on main, where the capture has already been destroyed. With this change, the full suite passes in Debug, and test_sleep / test_timer_stream are clean under ASan.