guest-diag: seed channel-event sets from the trace ring on client switch

clear_trace() on a pid switch dropped exactly the one-shot uvmt/uvch
events of the new client when they landed in the switch instant; the
pipe reader then never saw them and the whole session ran with an empty
sem set (r126: vecadd_loop sem=0 for minutes while its CE entries paid
the 8 s grace). Scan the ring tail once per switch and seed the sets
instead - stale kptrs that are also re-learned resolve to no content and
are deferred by the candidate scoring.

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