vgsp-gr r554: the per-VA launch-defer latch needs 64 slots, not 8

r553 soak evidence (victory_reduce, 430 s stall, 0 hangs): the same launch
push VA (0x20043be64) was deferred on every pass for 7+ minutes ("partial
argument set: 7-8 pages unresolved"). A client's UVM pool allocates ~8-10
channels and each command buffer reuses two launch push slots, so more
than 8 distinct push VAs rotate through the guard; with 8 latch slots each
new VA evicted another and the one-deferral-per-VA guarantee thrashed -
the second sighting of any VA found its record gone and deferred again,
so a partial argument set that never resolves held the launch forever.

64 slots cover a full client with margin (1 KB of static state).

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