Handling of trailing enqueues on realtime paths

[eliotheinrich]

Description

The current realtime path is inconsistent when the trailing measurements are not referenced by any detectors. Decoders fire once num_msyn_per_decode syndromes are accumulated, but if further syndromes are enqueued after this point, the syndrome buffer is corrupted, depending on the realtime path followed.

On the decoding server, any enqueue while shot_state == rseult_ready immediately reset the state to colleting, discarding pending correction result before the client can retrieve it.

On the in-process path (i.e. decoder::enqueue_syndrome), the decoder firing sets msyn_buffer_index = 0, so trailing measurements were written into the syndrome buffer at index 0. If reset_decoder is not called, the next shot is silently corrupted.

This PR fixes the decoding server path by introducing a total_circuit_measurements field to the decoder class, allowing for the decoder to absorb trailing measurements without disturbing the corrections result. The budget defaults to num_msyn_per_decode (i.e. no trailing measurements), and the caller can set it higher when circuits produce trailing syndromes. The total number of measurements is available as a field of the decoder context.

Runtime / performance impact

N/A

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