vgsp-gr r552: retire-wait settle window 300 -> 100 ms (launch p50 1119 -> 651 ms)HEADmaster

Measured A/B on real boots (same module, same client set, only
gsp_phys_gr_settle_ms varied):
 - r549 (default 300): 32 launches, min 862 / p50 1119 / p90 2363 ms;
   retire wait 680-901 ms per launch ("(settled)", 1..64 of 1000+
   tracked pages changed).
 - r551 (100): 22 launches, min 476 / p50 651.5 / p90 1010 / max 1428 ms;
   retire wait 231-247 ms. Battery 12/15, 0 hangs (unchanged profile;
   the 3 fails are the streams2/streams_events/memset2d readback class).

Why the wait was the dominant cost: a read-mostly kernel writes far fewer
pages than the launch's tracked set, so the "changed >= n_all" shortcut
never fires and every launch settles through the content-stable rule,
which paid the full window plus scan overhead. The stability verdict is
still hash-gated (m180); only the minimum stable duration shrinks, and
the battery correctness profile is unchanged. 300 remains available
through the module parameter.

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