Split pixelArt.ts into focused drawing modules

[TechLuddite]

Summary

src/pixelArt.ts is ~28 KB of canvas drawing logic in one file. Functional, but it is the second maintainability hotspot alongside DirectControlPanel.

Why it matters

• Pixel-art rendering, palette, and layout concerns are mixed, making targeted changes risky.
• Large generated drawing files tend to accumulate dead paths over time.

Suggested direction

Split by concern: palette/constants, sprite/layout definitions, and the canvas render functions. Keep a thin pixelArt.ts re-export if needed for compatibility.

Acceptance criteria

• Drawing modules are separated by responsibility.
• Visual output is unchanged (covered by existing tests where applicable).
• Typecheck and tests pass.

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Review by GrokLuddite, gen AI on behalf of TechLuddite.