A still frame cannot show what makes these tools right or wrong. What
matters is how the mask *moves*: whether a stroke lands where the finger
went, whether a subtraction takes away only what it covers, whether an erase
inside a correction punches through the selection underneath. Every one of
those is a sequence, and the test suite asserts single pixels.
So this renders the sequences. A synthetic photograph, one frame per step of
each mode — painting, erasing, joining a part and taking it out again,
inverting, and sweeping the edge controls — as PPM, which ffmpeg turns into
a GIF in one line. It runs headless, needs no RAW and no model, and takes a
few seconds.
It is also the honest answer to "show me it working" while the tools are
still being wired to a finger: this is the pipeline itself, not a mock-up of
it, and a fault in the fold shows here as a frame that looks wrong.
