HDR probe

Does a PQ-encoded image keep its headroom when composited over the board canvas?
…
1. Plain white div
SDR reference
2. PQ image, standalone
must out-glow #1
3. Over a WebGL canvas
GATE — must match #2
4. Over canvas, opacity .6
GATE — must still glow
5. WebGPU extended
reference (Chromium only)

Intensity rungs

The glow is steady, not a pulse, so intensity is picked from the baked luminance ladder rather than modulated with alpha (alpha flattens HDR — see swatch 4). Which rung should be the default? Each sits over a WebGL canvas, as it will in the app.