fix: ignore scale in directional light orientation

[besmpl]

Summary

• derive directional-light direction from composed local and ancestor rotations
• ignore translation and scale, including nonuniform, zero, and negative scale
• add local, parent, and degenerate-scale regressions

Why

DirectionalLight.Direction extracted a quaternion from the complete world matrix. The matrix's linear component also contains scale and hierarchy-induced shear, so changing a light or ancestor's nonuniform scale changed the reported direction even when every rotation stayed identical.

Node transforms use T * R * S. The new helper composes quaternions in the same parent-then-local order and rotates the default forward vector without allowing scale to contaminate orientation.

Verification

• baseline local and parent nonuniform-scale tests fail with distorted directions
• zero and negative scale remain rotation-defined instead of collapsing or flipping orientation
• nested rotation composition matches the world-matrix rotation when scales are identity
• reparenting and direct rotation updates are reflected immediately
• go test ./...
• go test -race ./...
• go build ./...
• go vet ./...
• Linux, Windows, and Darwin cross-builds
• gofmt and git diff --check

The helper recomputes the ancestor rotation chain when Direction is queried; this is O(hierarchy depth) and avoids coupling direction correctness to matrix cache state.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[kolkov]

Code reviewed: worldRotation() composes quaternions directly, bypassing world matrix — scale-independent by design. Unexported helper, correct recursion. 3 tests with non-uniform scale cases. LGTM.