guest-diag: cheaper trace read, optional push sampler, bigger pagemap batches

Under load the semfix poll grepped the whole (growing) trace file and the
track_waits push block spawned three guest processes per sample, both of
which slowed the pin path into the client's failure window. The trace read
now tails 128 KB; track_waits.sh gates the push dump behind PUSH=0; the
pagemap helper reads 32 KB chunks.

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