guest-diag: content-score CE mem candidates and refresh stale pin sets

The kptr-keyed one-shot latches assumed object identity, but UVM frees and
re-creates CE push-buffer rm_mems and channels mid-session and the slab
reuses the same addresses; a resolution taken before such a move pinned
pages that read all-zero when the host used them, so every CE ring entry
stalled in the 8 s grace (r125: the live pushes sat at new vmap pages of
the same rm_mem kptr). Candidates are now scored by their window content
(a live push buffer is full of method streams, a freed range reads zeros)
and the best is pinned - all-zero candidate sets emit nothing instead of
poisoning the host map. A resolved mem is re-validated every ~2 s against
its own {cpu,gpu} fields and first-page PA; when they move, the latches
drop and the current truth is re-emitted.

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