A node that has just been declared can be read, reasoned about and tested, and
none of that answers the question a photographer asks first: what does moving
this do to the picture. Colour grading, dehaze and the range masks were all
argued into the tree on their behaviour and none of them had been *looked* at.
So two diagnostics. `sweep` walks one operation from its minimum to its maximum
and writes a frame per step; `rangesweep` does the same for a mask band, which
is not an ordinary parameter — it lives on a layer, is rasterised by its own
pass, and only becomes visible through whatever adjustment the layer carries,
so it gets two stops down to make the selection legible.
`sweep` names no operation. The id arrives as a string and the parameters and
their ranges come from the graph's own capabilities, so a node declared
yesterday sweeps on the same terms as one that shipped a year ago — the
property `ops/README.md` promises, used rather than asserted.
Three things it learned the hard way and now records. It renders through
`render_detailed` unconditionally, because the fused path refuses a shader
composed with a detail stage rather than rendering it wrongly, and that call
falls through when there is no such stage. It takes `SWEEP_HOLD`, because a
parameter grouped under one widget is not meaningful alone: a hue with no
strength behind it renders the same frame every time, which reads as a broken
node rather than a correctly declared neutral. And it bounds the output, since
a 25 MP frame is a 75 MB PPM and a sweep is hundreds of them.
Both read a rendered file as readily as a raw one, so a JPEG can stand in where
no raw is to hand — on the terms `from_rgba8` documents, with the controls
still working and their neutral being what the camera left rather than what the
sensor recorded.
