Track observer position in ConstellationArtLayer and add unit tests

[kws]

Motivation

• Ensure the constellation art layer follows the observer's world position inside the content root so artwork stays colocated with the viewer while preserving the layer's orientation, and add tests to verify this behavior.

Description

• Added src/layers/__tests__/constellation-art-layer.test.js with two tests that verify the layer tracks observer position and that orientation is not changed when the observer moves.
• Updated src/layers/constellation-art-layer.js to compute the observer's world position and convert it into contentRoot local coordinates, setting the layer group.position accordingly.
• Introduced internal vectors _observerWorldPosition and _observerInContentPosition to avoid reallocations during update.
• Left existing opacity/fade logic unchanged and preserved existing public API (getConfig, attach, dispose).

Testing

• Executed the new unit tests in src/layers/__tests__/constellation-art-layer.test.js using the Node test runner via node --test src/layers/__tests__/constellation-art-layer.test.js.
• Both tests passed, confirming position tracking and orientation preservation.

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Codex Task