Explorer UI: wheel-zoom, reliable background alternation, effects as tiny text

Desktop mouse wheel now zooms the star map toward the cursor (no ctrl needed; deltaMode-normalised). ctrl+wheel / pinch still work.

• Background "video+demos" mode actually alternates now: a wall-clock backstop (~16s) drives it whether live or idle, so it no longer depends on onBar firing and no longer imperceptibly slow.

• The ⓘ timeline's effect labels were stacked lozenges that clipped to one visible; now the FULL chain renders as one tiny " · "-joined line under each lane (and the master line), tightening the panel.

Verified: explorer-ui (adds wheel-zoom + clock-alternation + layer-tracking asserts), genre-viz (asserts the full fx line, no stacked pills).

▶ Play it: Mercury Interrogation — where the harmony organ has the least anywhere to hide https://stellate.app/?genre=bebop&seed=8242&path=694.13834,713.13867,675.13867&xdur=13.122

adffe73 https://github.com/aboard-io/stellate/commit/adffe73ab7858869867665807d06db6584bf22ee