boot/gr: r391d post-write witness - read the VA's landing page immediately after the copy-out write

The sum=0 class: OUT-PAGE-WIT shows the changed page's bytes in the bounce
(the correct sum) and the ledger resolution (pa=0x7c00000 ap=FB), the
copy-out writes hpa=0x16f07a000, and the D2H read of the same ledger page
8-34 ms later reads zeros. sema-fbhpa proves __va(hpa) == base+pa at submit
time. This witness reads the page right after the memcpy through the same
path the D2H reader uses, so the next failing run names whether the write
target and the reader's page differ (aperture/staleness) or the write
vanished.

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