Add tests for the logo subsystem

[jonfroehlich]

From the June 2026 audit. The rest of the library now has solid coverage (95 cases across math/graphics/serial/array-utils), but the src/lib/logo/ subsystem (~4,500 lines: makelab-logo.js, makelab-logo-morpher.js, triangle-art.js, the intro animations) has zero tests.

What to cover

• Pure geometry: Triangle/Cell/Grid construction, directions, bounds.
• TriangleArt morph-target representation.
• Color palette + MakeabilityLabLogoColorer.
• morph-paths.js (linearPath/arcPath/bezierPath/spiralPath).
• draw() methods can be smoke-tested against a mock 2D context (no real canvas needed).

Notes

• Uses the existing zero-dependency runner in test/ (see test/README.md).
• Good patterns to copy: test/line-segment.test.js (mock CanvasRenderingContext2D) and test/serial.test.js (mock-backed, environment-aware via skip()).
• Likely pairs well with Get webpack working #1 (the split) — testing the extracted primitives is easier than the monolith.