guest-diag: sparse allocation-head scan in the pin stream

The first deltas of a 15k-page window took tens of seconds to arrive while
the client was mapping under load, so the pins still landed after the
client's cuCtxCreate deadline. The scanner now reads only the first 64 KB
of every 2 MB region - the pushes sit at the per-channel allocation heads -
which cuts the per-loop page-table work ~50x, and it logs scan timing so
the latency stays visible.

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