fix: size the DVD logo from the short axis, so it stops being huge on the wall (#117)

[BernardJen]

Refs #117 — addresses the size complaint folded into that issue. Its original items (motion
blur, bloom, an actual corner hit) are untouched, so it stays open.

The complaint

The logo is too big.

Measured at 6000×1200 before this change: 871 × 238 px, close to a metre across on the real
wall.

The axis was wrong, not just the number

The logo was sized as a fraction of canvas width, with height derived to preserve its pixel
aspect:

logoW = 0.10..0.15 of width
logoH = logoW / aspect * (width / height)

The derivation is correct. Anchoring to the width is not. On a 5:1 canvas, 12% of 6000px is
720px wide — and the derived height follows it up until the logo spans a fifth of the frame's
height.

Reversed: height is now chosen from the short axis and width derived. That is what every
other saver in this folder does, for the same reason — pointScale and particleSide in
gl-base.js, CELL_PX in ascii-donut.js, the cell sizing in truchet.js. A size in angular
terms has to come from the dimension that does not stretch.

Measured, on both aspects

Canvas	Before	After
6000×1200 (the wall)	871 × 238 px — 19.8% of height	519 × 143 px — 11.9%
1920×1080	280 × 76 px — 7.0% of height	466 × 126 px — 11.7%

The wall figure is what was asked for: 40% smaller linearly, 63% less ink.

The 16:9 figure goes the other way, and that is the same bug rather than a regression. The old
sizing made apparent size depend on the canvas shape — 19.8% of height at 5:1 against 7.0% at
16:9, a 2.8× swing from aspect alone. It was too big on the wall and a speck on a laptop for
one single reason. After the change both read ~11.8%, which is the entire point.

If you would rather 16:9 keep its old smaller logo, that needs a second size anchored to
something else and I would argue against it — it reintroduces exactly the shape-dependence this
removes.

It has been fixed once before

"DVD logo screensaver drawn smaller" — v2.6.2. It came back because the size stayed anchored to
the width, so widening the canvas re-grew it. Anchored to the short axis, it cannot.

Verification

• Branched from main (926c4ef).
• npm run lint clean, npm test — 416 passed.
• npm run shadercheck — ok=150 fail=0, no baseline change. DVD Logo's measured density
  rose from 0.0026 to 0.0065 rather than falling, because the harness canvas is near-16:9 where
  this change makes the logo larger. The check only fails on drops, and 2.5× is inside the
  staleness threshold added in fix: report stale structure baselines, and correct three of them (#227) #234. The sub-margin set is unchanged — still Plasma alone, as its
  test asserts.
• Rendered and measured at 6000×1200 and 1920×1080, same seed, before and after.

Not verified

• Whether ~11.8% of height is the size you actually want. It is roughly half the wall's
  previous linear size, which is what "too big" implied, but the target was my choice —
  LOGO_H_RANGE = [0.09, 0.12] is one constant to change if it should be smaller still.
• Not reviewed in motion. The bounce, speed and hue-on-contact behaviour are untouched, so there
  is no reason to expect a change, but I have only looked at stills.