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/decisions/0009-responsive-ultrawide-work-lanes.md

@@ -17,8 +17,14 @@ independently of selection.
 Terminal geometry is computed from the width of one panel and the height of its
 middle 60% region, including gaps and content insets. The selected real PTY is
 resized through the runtime actor when the responsive panel count or window
-bounds change. On the 3440×1440 reference display the current fixed-cell spike
-derives approximately 71 columns by 42 rows per panel.
+bounds change. Cell size is **measured**, not assumed: the advance of `0` in the terminal face
+comes from the text system, so the column arithmetic and the painted glyph agree
+by construction. This paragraph previously quoted "approximately 71 columns by
+42 rows per panel", derived from a hardcoded 8.4 px advance that nothing had
+ever measured. The real advance is about 7.3 px, so those numbers were roughly
+13% short — one full-width pane on the reference display measures 140 columns,
+not 122. Any figure quoted here is a consequence of the measurement and the
+current panel count, and will move when either does.
 
 The runtime slice now owns three independent real PTYs through a pane-indexed
 session registry, one for every terminal fixture. The Markdown, browser, and