perf: replace per-block SMIL animate with shared CSS keyframes in rainbow theme

[K-H-A-L-I-D]

Problem

profile-night-rainbow.svg weighs ~375KB, more than double any other theme. The culprit is addRainbowColor() in create-3d-contrib.ts. It appends an inline SMIL <animate> element to every rect face, and with ~364 blocks at 3 faces each, that produces ~1,092 animate elements. Each one repeats 7 full RGB color-stop strings. About 240KB of the file is nothing but duplicated animation markup, which makes the SVG slow to parse and noticeably sluggish when embedded in a profile README.

Solution

Every rect gets a CSS class (rb-l{level}-{top|left|right}) and an inline animation-delay that encodes the week-based hue phase offset. That preserves the per-column rainbow wave. The 15 @keyframes definitions (5 contribution levels times 3 faces) live once in the SVG <style> block instead of being duplicated across every element.

The delay that maps each block to the right starting phase:

delaySeconds = -((week * hueRatio % 360 + 360) % 360 / 360) * durationSeconds

Result

	Before	After
Animation elements	~1,092 <animate> tags	15 @keyframes + 15 class rules
Animation overhead	~240KB	~5KB
Total SVG size	~375KB	~140–150KB
Visual output	Per-block rainbow cycle	Identical

Changes

addRainbowColor() in src/create-3d-contrib.ts now sets a CSS class and animation-delay instead of appending <animate>. A new exported function getRainbowKeyframes() generates the 15 keyframe definitions and lives in the same file. src/create-svg.ts injects that output into the SVG <style> block when settings.type === 'rainbow'.