sema: opdef absolute per-launch hold bound (GRFIX lesson) + physical guest-RAM fill branch + diagnostics

The opdef hold used a slot-keyed deadline that a pin-batch rewind
re-arms on every replay (observed opdef=3301/32/0 with the guest
wedged); the bound is now keyed to the launch push VA and written
back through the pump snapshot, like the GR hold bound (r213).

The fill op builder also accepts physical SYS destinations below the
VA floor (guest-RAM page-table clears; bounded by SEMA_PA_MAX).
The vidmem-window zero-fill experiment (r362) was reverted: executing
those fills with elem=1 wrote 4-byte partial zeros and broke results
(no drops but sum=0); the window fills need their true element size
and value semantics decoded first (see handoff).

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