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).

1 files changed, 14 insertions, 5 deletions

diff --git a/CONTEXT.md b/CONTEXT.md
index f068035..bf1e01f 100644
--- a/CONTEXT.md
+++ b/CONTEXT.md
@@ -113,13 +113,17 @@ Workspace version is `a.b.c` (single version for all crates):
 
 Bump the workspace version in the root `Cargo.toml` when a change of
 that class lands — do not leave the version to drift.  Current:
-`0.5.2` — the GUI era landed with 0.5.0 (graphics layer + cursor
+`0.5.3` — the GUI era landed with 0.5.0 (graphics layer + cursor
 compositor); 0.5.1 fixed the canvas at 800×600; 0.5.2 added fault
 diagnostics (screen + serial dumps for every CPU exception) and
 real-hardware hardening (framebuffer-size clamping, fb-region exclusion
-from the allocators, authoritative fb geometry logging).  A future
-major GUI overhaul (graphics windows beyond the text surface) would be
-the `a` bump (1.0.0).
+from the allocators, authoritative fb geometry logging); 0.5.3 fixed
+the real-hardware scroll wedge — the scroll's full-frame push is now
+per-word volatile stores like the boot-time `clear()` instead of one
+1.92 MB rep-movsb, which can stall forever on an uncached real-GPU
+framebuffer (a stall, not an exception — hence no crash dump).  A
+future major GUI overhaul (graphics windows beyond the text surface)
+would be the `a` bump (1.0.0).
 
 ## Project layout
 
@@ -553,7 +557,12 @@ Workspace of four crates, all built for the `x86_64-unknown-uefi` target:
      shadow too; scrolling is a cached shadow memcpy + one write-only
      push to the display — the display is never *read* (uncached reads
      were the killer, since real-hardware GOP framebuffers are
-     uncacheable/write-combining).
+     uncacheable/write-combining).  **The push is per-word volatile
+     stores (0.5.3), not one bulk rep-movsb**: a 1.92 MB REP MOVS to an
+     uncached real-GPU framebuffer wedged the CPU on some hardware (a
+     stall, not an exception — no dump fired), while the boot-time
+     `clear()` — the same per-word pattern — always worked.  QEMU's
+     RAM-backed framebuffer never showed it.
   2. deferred scroll batching (`vt::PENDING_SCROLLS`): the logical grid
      scrolls instantly in RAM while the framebuffer shift+push is
      deferred; N pending scrolls cost one push.  Bursts of streaming