rsnix v0.5.3: fix real-hardware scroll wedge — per-word display push

The real-hardware crash (2 lines of white + freeze, no CPU exception
even with the 0.5.2 fault dumps) pointed away from a fault and at a
stall: the scroll's full-frame push used one 1.92 MB rep-movsb to the
framebuffer.  On QEMU the framebuffer is RAM, so it flies; on a real
GPU the framebuffer is uncached MMIO and the CPU's fast-string burst
path can stall forever — the shell wedges mid-instruction, the cursor
task's own writes wedge too, no exception fires (MMIO writes don't
fault), and the screen shows the torn partial push (the white bands).

The boot-time clear() always worked on the same hardware and uses
per-word volatile stores, so the scroll push now uses the identical
pattern: shadow reads (cached RAM, cheap) + individually-posted
display stores.  Interruptible between stores, never a bulk burst,
and functionally identical on QEMU (all 5 suites re-pass).

Also converts the no-shadow fallback shift to the same per-word
pattern.  CONTEXT.md updated (scroll-cost note + versioning).

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