vgsp-gr r553: the dstpend launch-defer window is absolute, not sliding

r551 soak evidence (victory_strided, same boot as a green battery): the
client's init copies keep failing -ENOENT and are retried by the pump;
every retry re-noted its destination page and re-freshened the r541
dstpend record, so svc_gr_dstpend_overlap() never lapsed and the launch
guard deferred the launch forever: "GR busy-defer timeout -> hold
8000 ms" every ~4 s for 427 s, zero GR LAUNCH submitted, client at 100%
CPU.

Stamp a page's record once per failure episode: an existing record inside
its 10 s window is not re-stamped (the window stays absolute from the first
unexecuted write); an expired record is re-stamped for a genuinely new
episode. The launch then forwards best-effort at the bound exactly as the
guard's own comment promises ("bounded by the same busy-defer + gr_hold
machinery ... it can never hang the launch").

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