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, 3 insertions, 3 deletions

diff --git a/Cargo.lock b/Cargo.lock
index 942456c..65166d4 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -4,7 +4,7 @@ version = 4
 
 [[package]]
 name = "rsdos"
-version = "0.5.2"
+version = "0.5.3"
 dependencies = [
  "rsnix",
  "shell",
@@ -19,11 +19,11 @@ dependencies = [
 
 [[package]]
 name = "rsnix"
-version = "0.5.2"
+version = "0.5.3"
 
 [[package]]
 name = "shell"
-version = "0.5.2"
+version = "0.5.3"
 dependencies = [
  "rsdosedit",
  "rsnix",