vgsp r580: walk-over-bootcache for pool pages + event burst dedup and fast-path hint

Pool resolution (thread: the starve wedge surviving the r577 fix): the
boot-scoped pool cache survives session flushes, but a pool page MOVES
between sessions and the old page often stays readable, so the read-back
proof cannot tell - the resolver wrote releases to the stale cache PA while
the client's tracker read its current page. Live evidence (r579 boot):
pool-wr src=bootcache pa=0x11e6d9000 for va=0x121000000 while the injected
pin/current page was pa=0x134730000; the client spun in uvm_tracker_wait
for minutes. svc_pool_pa_resolve now tries the structural walk (the
client's own page tables) BEFORE trusting the cache: a differing,
read-proven walk page wins (src=walk-over-boot) and the cached page is kept
as the fan-out secondary.

Event burst (r579 boot: events_dropped=724, the 'no postable event
(events_n=8 dropped=724)' sentinel every 0.5 s): apply the 250 ms per-event
dedup to the per-completion shotgun (8 records per completion burned the
48/s budget), and hoist the completion hint above the pump fast path so it
runs under load too (a dropped wake is re-posted within 0.5 s).

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