Implement `timeout`

[sgerbino]

timeout

Fail a task if it doesn't complete within a duration.

Behavior:

• Starts the task and a timer concurrently
• If task completes first, returns its result
• If timer fires first, requests stop for the task and returns error condition capy::cond::timeout

Real-world use cases:

Use Case	Description
SLA enforcement	Ensure responses within promised latency
Deadlock prevention	Abort operations that hang indefinitely
Resource protection	Prevent slow clients from holding resources
User experience	Show timeout message rather than hanging UI

auto[ec, n] = co_await timeout( sock.read_some( buf ), 50ms );