perf(renderer): one frame clock and pacer, fewer style writes, decode before a mesh reveals

[alowpoly]

iPad stress runs found long frames and wasted style writes.

• A hidden mesh decodes its images off the main thread before it shows again: Mars's 177 ms reveal frame is gone.
• A document has one frame clock and one paced budget a frame, shared by every paced owner. The longest busy frame of the Mars run fell from 80 ms to 57 ms.
• No write sets a value CSS already holds. Invisible opacities are written once, as 0. Keyboard and accessibility state waits for the camera to stop.
• Every catalogue dot draws as SVG; the box-shadow painter is gone. A camera turn warps the painted dots with one transform, and they repaint exactly at the pause; which dots a zoom draws lands there too.
• Leaf backgrounds are shares of the box, so a leaf-box step writes only the box and its transform.
• Galaxy layers (backing, ring sections, dot layers, large volume and image-layer slices) switch on one a frame, their images decoded off the main thread first: Saturn 99 → 57 ms, Jupiter 97 → 53, the Sun 88 → 51, M31 309 → 128 ms longest frame.
• Orbit trails fade in eight steps instead of sixteen (pixelmatch 0 on the rested Solar System).

Three rest views stay pixel-identical (pixelmatch 0).

Limits: one iPad run per change; write counts vary about 5% between identical runs.

Clean: renderer typecheck and tests, affected site tests, eslint.