Add Vitest (test:unit) with exact-value tests for the crop math (containScale, defaultSelection, clampSelection, all resizeSelection corners, and mime/filename resolution), and wire it plus the Pest browser test into CI. The browser test now samples output pixels against a four-quadrant fixture, so it detects a blank, flipped, or wrong crop rather than only asserting a 512x512 canvas. Add composer test:all to run PHP + Vitest + browser tests locally.
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