style(design): FINDING-005 — the viewport field reads the shell tokens

The largest surface in the product was painted from the old palette in five places:
the shell's background (`--cad-bg` #0A0D10 / #F4F6F8), its straight grid
(`--cad-grid`), a 14% accent radial halo behind the part, a 1px legacy border with an
8px radius, and in three.js the scene background, the fog (#0B0F14, which is what the
user actually sees as the field) and the GridHelper's cool blue-grey lines
(#2B3643 / #C9C3B8). The canvas is transparent, so the DOM colour decides the field.

Field colour, fog and grid now come from `--shell-canvas-deep`,
`--shell-field-grid` and `--shell-field-grid-strong`, which the viewer reads at
runtime; the halo and the inset frame are gone (DESIGN.md section 3 makes the canvas
full-bleed, section 4 puts the light on the camera). Measured field colour: (11,15,20)
before, (14,13,12) after, matching canvas-deep-dark.