Module scene/lod-selector-math

Selector math for the per-frame LOD-group selector.

The camera-geometry half of lod-group-registry.ts, extracted so the registry file holds the policy/state machine and this module holds the (individually unit-testable) math:

• computeEntryWorldBox — fold a group's per-child nD raw or robust bounds into one world-space box via displayDims + matrixWorld.
• projectBoxDiagonalPx — project that box through the camera to a screen-space pixel diagonal (with near-plane saturation). The legacy selector: 'coverage' metric.
• projectBoxAreaFraction — project that box to the fraction of the viewport AREA its screen-space rect covers (same near-plane saturation). The selector: 'screen-area' metric.
• pickChildWithHysteresis — pick the level for whichever metric the group's selector names, with asymmetric downgrade hysteresis.

The registry re-exports projectBoxAreaFraction / projectBoxDiagonalPx / pickChildWithHysteresis so existing importers (the selector unit tests) are unchanged.