fix: the stage's controls track was eating the field

[DoodleBears]

Why

Reported twice as "Column Drop shows nothing". The previous fix (#18) was correct and
insufficient: the motion was rendering — 25 cells, animating, one column lit at a time. It
was being clipped out of the viewport.

Change outline

The stage is grid-template-columns: auto minmax(0, 1fr), and an auto track is sized by
max-content. The refusal caption — "this motion is a highlight moving down the field…",
shown only when the CSS renderer refuses the chosen motion — is a sentence, so that one
caption grew the controls track from 264px to 602px and collapsed the field's track to
zero. Measured in Chrome, before:

	stage	controls	field
spiral (caption hidden)	648	264	338
columns (caption shown)	648	602	0

The stage is overflow: hidden, so a zero-width track doesn't squash the field — it
removes it from the render. The DOM held all 25 cells with correct fill-opacity; the
page showed nothing.

One declaration, verified in a browser (after: 264px / 338px in both states):

grid-template-columns: minmax(0, 264px) minmax(0, 1fr);

Special things to note

• Reversibility: one CSS declaration. Review focus is whether 264px is the right cap —
  the capping is the fix, the number is a judgement about how wide the steppers should be.
• No automated test for this one, deliberately — and that is the interesting part.
  • jsdom computes no layout: getComputedStyle(el).gridTemplateColumns on a real grid
    rule returns the empty string. Nothing in this repo can catch a collapsed track.
  • I wrote one anyway. It regexed the stylesheet for the rule and matched a different
    component's repeat(auto-fill, minmax(280px, 1fr)) — so it went green with the bug
    present, and would have stayed green forever.
  • A test that can assert about the wrong rule is not a slow test, it is a wrong one,
    and a green wrong test costs more than no test, because it is believed.
  • AGENTS.md records the class instead: layout must be verified in a real browser over CDP,
    with a dedicated --user-data-dir (reusing the default profile makes the Chrome instance
    exit immediately — that cost more time than the bug did) and screenshots.
• 215 tests (was 215), oxlint 0, registry ok, both changelog ledgers.

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