The option tells points from picas, where a bare `12` could be either: the
reference app dims it unless a unit is Picas, and it starts on. A number typed
with no unit into a field in picas is now read in points with it on (the
default again, as before #486), and in picas with it off; fields in any other
unit read it in their unit either way, as there. The Preferences dialog dims
the checkbox unless one of the Units is Picas.
The fields learn the preference when it changes, through
`prefs_dialog::apply_runtime` as Scrub Numeric Fields does, rather than from a
write into the egui context every frame. Tests for picas and other fields, the
wiring and the dimming; docs/mcp.md and ROADMAP say so.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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