FABRIC.md item 4.4z: draw the scroll box's visible border

draw_box_border() (vt100.c) strokes four 1px edges around the 640x480
CANVAS box, reusing the same border-gray constant the REPL strip's
border lines use (renamed VT100_STRIP_BORDER_GRAY -> VT100_BORDER_GRAY
since it's now shared -- one pinned color decision, 4.4w, not two).
Called from erase_display()'s box-scoped branch so the border survives
every box clear (the initial one and any later ESC[2J), not just the
first.

Verified: three-arch clean QEMU boot + logs, amd64 screendump showing a
full rectangle outline around the box, visually distinct from the strip
below it.

Punch list §25 item 4.4z complete.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

logs/screendump-4.4z/qemu-stdout.log

@@ -0,0 +1 @@
+qemu: terminating on signal 15 from pid 102222 (/bin/bash)