FABRIC.md item 4.4t: confine REPL text rendering to the CANVAS box

vt100's TTF-mode text grid now operates within the per-arch 640x480 box
(computed in 4.4o, pixel-verified in 4.4p) instead of the full framebuffer:
box-origin offset in px_of()/py_of(), box-derived cols/rows (53x20) set
before the 4.4q scrollback allocation depends on them, mode-aware
erase_display()/reverse-index fill, and a new box-scoped fb_scroll_rect()
alongside the existing whole-framebuffer fb_scroll_rows() (bitmap/boot mode
unaffected either way). Also clears the full framebuffer once at the
bitmap-to-TTF switch so leftover boot debris doesn't sit frozen outside the
box now that erase_display(2) is box-scoped afterward.

Three-arch QEMU boot + pixel-scanned screendumps confirm zero non-background
pixels land outside the box on amd64, aarch64, and riscv64.

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

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