feat(svg): add single-outline symbol for CAD import

[IamMrCupp]

Closes #32. Part of #26 (Phase 3 — extension).

The canonical audiophore-symbol.svg is 13 separate <rect>s, each with its own spectrum color and a partial opacity for the depth ramp. Both are meaningless in CAD — and worse, OpenSCAD's import() treats the multi-color, multi-element art as something it can't cleanly extrude. So the hardware repo hand-rebuilds the 13-pixel phi as a diamond_pixels cell list in SCAD, a parallel copy that drifts from the brand spec.

This emits a flattened companion — logos/svg/audiophore-symbol-solid.svg — where the lit cells are one <path> (one rounded-rect subpath per cell), a single flat fill, no opacity, same 320-unit viewBox as the canonical mark. import() + linear_extrude gives one clean solid; hardware imports the canonical geometry instead of maintaining its own.

New [cad] section in brand.toml (just the flat fill key); folded into the symbol build step so make svgs/build produces it. Same grid math as the canonical symbol, so the silhouette is identical. Byte-identical on rebuild.