Module data/scene-loader/nodes/load-lod-group-node

Initial-load path for a single lod_group scene-graph node.

Mirrors the generic-group branch in load-scene-nodes.ts — creates a THREE.Group, applies the transform, recurses into children — with two additions:

1. Each child carries a coverage_fraction attribute (a per-child threshold whose UNITS the group's selector attr names: literal screen-area fractions under 'screen-area' — derived whole-object ladders span [0, 1/2], a partition tile anchors at 1.0 — or the legacy diagonal metric under 'coverage', up to 4.0 == SCREEN_FILL_DIAGONAL_RATIO / FILL_FACTOR) plus its own position_bounds (the raw nD AABB) and optional lod_bounds (a robust selector-only AABB). All are read from the child's zarr attrs. Legacy (pre-v3.2) datasets that still carry min_pixel_size / selector 'pixel_size' are auto-adapted with a warning (see resolveCoverageFractions).
2. Once children are loaded, an entry is registered with the :class:LODGroupRegistry so the per-frame selector can pick which child renders.

The lod_group walks the scene-graph child order (which the Python writer guarantees is coarsest→finest); the registry stores children in that same order for consistent threshold comparisons.

Lazy loading: only the default level's geometry is loaded eagerly. Every other gsplats, points, lines or mesh level is cheap-attached (placeholder + loader, no array fetch) with an ensureLoaded thunk; the registry fires the thunk on demand the first time the per-frame selector wants to show that level. This is what keeps a scene of many lod_groups from loading every level of every group up front — distant groups stay coarse and their fine levels are never fetched. Deferring the points/lines/mesh level matters for their substitutive ladders, whose finest child is the full cloud / line set / full-resolution surface (eager-loading it would defeat progressive loading). The selector math needs only the per-child coverage_fraction / position_bounds / optional lod_bounds attrs (read here), not loaded geometry, so deferral is fully correct.

Sibling of data/scene-loader/nodes/load-scene-nodes.ts (dispatch), data/scene-loader/nodes/load-gsplats-node.ts + data/scene-loader/nodes/load-points-node.ts (cheap/expensive splits), and scene/lod-group-registry.ts (per-frame eval + lazy trigger).