Correct the column figure decision 0009 quoted from a guessed cell width

"Approximately 71 columns by 42 rows per panel" was derived from a hardcoded
8.4 px advance that nothing had measured. The measured advance is about 7.3 px,
so the figure was roughly 13% short — a full-width pane on the reference display
measures 140 columns, not 122. The paragraph now says the number is a
consequence of the measurement rather than a property of the design.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/ARCHITECTURE.md

@@ -71,6 +71,12 @@ clipboard writes are disabled by default. The renderer never imports the
 upstream terminal type, and protocol replies return through the same actor input
 queue as human input. See decision 0005.
 
+Terminal cell size is measured from the text system rather than assumed, and the
+viewport's pixel extent is reported to the PTY. Both were wrong before: the
+advance was a hardcoded guess 13% wider than the face actually painted, and
+`ws_xpixel`/`ws_ypixel` were zero, so every program doing pixel arithmetic was
+told the window had no size.
+
 Each `lumbridge-runtime` actor owns one `PtySession` on a dedicated thread.
 Bounded command and event queues serialize input, resize, close, and shutdown
 against ordered raw-byte output. A generic pane-indexed registry now owns and