#42 Add hvc 6 yield runtime hook

Cooperative yield request from the HVC 6 handler. When set, the current guest run loop exits after the handler returns and control is yielded back to the host.

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Summary by cubic

Add a cooperative yield path for HVC 6 so the handler can request the vCPU run loop to exit and return control to the host right after it returns. Introduces a thread-local runtime hook via proc_request_hvc6_yield() that vcpu_run_loop clears before the HVC 6 handler and checks after; when set, the loop stops immediately after the handler returns.