FABRIC.md: item 4.4p -- CANVAS geometry verified via one-shot probe, all three architectures

Verification only, no permanent code (matches this item's own title
and 4.4i's same-shape precedent -- 4.4l-4.4o were geometry decisions,
not drawing code). One-shot diagnostic probe in sk_repl() drew 4.4o's
box outline plus a strip-top marker, then was reverted after capture
per this document's write/run-once/capture/revert discipline.

Measured pixel bounds directly from each screendump (not eyeballed)
and confirmed exact matches against 4.4o's computed coordinates on
all three architectures: amd64 box x:[320,959] y:[104,583], strip
marker y:704; aarch64/riscv64 box x:[80,719] y:[4,483], strip marker
y:504.

Observed, not fixed here: on the small architectures the REPL banner
text visibly overlaps the box's top edge, because vt100's cursor grid
still spans the whole screen rather than being confined to the 96px
strip -- a real gap between the landed REPL path and the mockup,
belongs to 4.4q/4.4r's wiring work, not this item.

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

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