Two gaps in the commit before this one.

`--selftest` writes no state file, so `_paint_state` returned at its first line
through all three themes and all four window shapes. The previous message cited
that PASS next to the new layer, which reads as coverage it did not have: the
drawing had executed in exactly one place, a throwaway script, which is how a
whole layer stays green while never running. There is now a differential render
test — paint offscreen, read the pixels back, assert WHICH SIDE of the detent
filled: left for a low filter, right for a high one, neither for one at rest,
and nothing at all in the five columns this slice does not own.

Getting that test right took three attempts, all three failing on the widget
rather than the layer. Asserting two x positions in a cell are equal is never
safe here — the CRT texture draws vertical lines every few pixels — and the
strip's TRACK spans the full cell, so "this pixel changed" is true across the
whole row regardless of value. The version that holds blits the texture off (the
one overlay not under test) and compares flanks, plus asserts low and high are
mirror images of each other rather than merely different.

Second: the zero zone was a bare `abs(val - 64) <= 2`. It is now ZERO_ZONE, at
module scope, with the reason it is a guess written next to it — rows B/C are
relative and the driver integrates with a clamp (`max(0, min(127, cur + d))`),
so a knob swept to an end and brought back loses every click it spent against
the clamp and settles NEAR 64 rather than on it. Too tight shows a sliver of LOW
where his hand says ZERO; too loose hides a real nudge off centre, which is the
worse failure because the layer exists to stop the picture lying about rest.
Narrow to start, and it wants a hand on the board rather than another argument.

Suites 529 passed, 2 failed (both pre-existing).