D281 — The reading panel stops growing at its prose measure

Decision. The reading panel's upper bound (D239) is the smaller of what leaves 280px of graph beside the rail and what its content can use: the concept's reading measure (--reading-measure, 68ch, the .markdown max-width) plus the body's padding and an allowance for the border and a scrollbar. App.tsx reads that width off a hidden probe styled with the prose's own font, so the bound follows whatever font the browser resolved; while unmeasured it falls back to the room alone. When the panel's width changes under a selection, the graph reframes the selection in the strip that is left, once the width holds still for 150ms, without the signals a new selection sends.

Why. Past the measure a wider panel draws an empty band on its right and takes graph the reader then cannot use: on a wide window the graph could shrink to 280px behind a panel whose text stayed 68ch wide. Readers took it for "text not responsive" and "graph unusable" (#436). Bounding the panel bounds the cost D239 accepted — "a wide panel hides more of a graph that is still drawn underneath it" — to the width that buys something. The cost: tables and code blocks keep the prose's width even on a very wide window.

Alternatives rejected. - Letting tables and code blocks use the extra width while prose keeps 68ch: it leaves the empty band on every concept with neither, and it keeps the graph squeezable to 280px for them too. - A fixed pixel cap: 68ch is 544px in one serif and more in Georgia, so a constant is either too tight on one platform or leaves a band on another. - Re-framing on every pointer move of a drag: each move restarts the camera tween, so the node chases the handle; the debounce frames it once it settles.

Consequences. The stored preference is never rewritten by the cap (D239's rule for a window resize holds for it too): a width stored before this change is drawn clamped. A change to the prose measure changes --reading-measure in tokens.css, never a literal in .markdown or in the probe.