Implement viTerminate() for HiSLIP sessions (cross-thread I/O cancellation)

[jjgilles99]

pyvisa-py has no implementation of viTerminate() — the VisaLibraryBase.terminate() method raises NotImplementedError. For HiSLIP sessions, this means there is no way to cancel a pending I/O operation from another thread.

1. A SCPI command like *WAI or a binary data query can block for seconds to minutes while the instrument completes an operation.
2. The thread blocked in socket.recv_into() inside hislip.Instrument.receive() cannot be interrupted.
3. The only workaround is socket.shutdown(SHUT_RDWR) from another thread.

The VISA spec defines viTerminate() to handle exactly this case: cancel a pending synchronous operation so it returns VI_ERROR_ABORT, without destroying the session.

Proposed solution

Implement viTerminate() for TCPIPInstrHiSLIP using a cancellable socket wrapper.

CancellableSocket

Wrap the HiSLIP sync socket so that recv_into() uses select() to watch both the
data socket and a cancel pipe (socket.socketpair()). When cancel() is called from
another thread, the blocked select() returns immediately.

class CancellableSocket:
    def __init__(self, sock):
        self._sock = sock
        self._cancel_r, self._cancel_w = socket.socketpair()
        ...

    def recv_into(self, buffer, nbytes=0, flags=0):
        readable, _, _ = select.select(
            [self._sock, self._cancel_r], [], [], timeout
        )
        if self._cancel_r in readable:
            self._drain_cancel()
            raise HiSLIPInterruptedError(0)
        return self._sock.recv_into(buffer, nbytes, flags)

    def cancel(self):
        self._cancel_w.send(b"\x00")

This is a drop-in replacement — all existing code that calls sock.recv_into() (including receive_exact_into, RxHeader, receive_flush) gets cancellation support transparently.