The rail paint is extracted to components/confidence-rail.tsx (ramp,
tick stack, geometry laws) and shared by both surfaces so they cannot
drift: the dock's live slider paints the rail under its disc, the
answered card renders it disc-less — the missing handle itself says
'record, not control'. Same look otherwise: thin rail, stop color,
solid ticks, % read-out on the right, dock-mirrored placement (below
the options, above the thinking).
Renders only where a confidence exists: quiz answers with one sent;
never on 'I don't know' (none was sent), never on explain/ask, never
on pre-feature records (optional field, absent).
