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, 5 insertions, 1 deletions

diff --git a/rsnix/src/sched.rs b/rsnix/src/sched.rs
index cfce58f..f48e210 100644
--- a/rsnix/src/sched.rs
+++ b/rsnix/src/sched.rs
@@ -282,7 +282,11 @@ pub fn spawn(entry: fn(), name: &str) -> Option<usize> {
     };
     let ok = spawn_at(id, entry, name);
     unlock();
-    if ok { Some(id) } else { None }
+    if ok {
+        Some(id)
+    } else {
+        None
+    }
 }
 
 unsafe fn reap(id: usize) {