fix: CPU AA rasterizer quality improvements (#95)#127
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AdvanceX() advanced line.X by one sub-pixel step per pixel scanline, but computeSegmentCoverage already computes positions analytically via lineTopX = x + dx*(lineTopY - firstY). The accumulated drift caused progressive edge position error (slope * N / aaScale per scanline), making triangle edges visibly expand toward the bottom.
coverageToRuns used AlphaRuns.Add() which sets maxValue=255 for middle pixels in merged runs. When two adjacent pixels had identical partial coverage (e.g. 44% at triangle vertices), the second pixel got 100% instead of 44%. Fixed by using AddWithCoverage(maxValue=currentAlpha). Added 4 regression tests: - TestVertexCoverage_PartialAlpha: vertex pixels get correct partial alpha - TestEdgePosition_NoDrift: edges don't expand over distance - TestCoverageToRuns_MergedRunAlpha: merged runs preserve alpha - TestInteriorPixels_FullCoverage: interior pixels still get 100%
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Summary
Closes #95