Function build_parent_map 

pub fn build_parent_map(
    main: &mut World,
) -> (HashMap<Entity, Entity>, HashMap<Entity, u32>)

Returns (parent_map, document_order_map) used by every extract fn to compute PaintOrder consistently.

• parent_map: child -> parent for every entity carrying bevy_ecs::hierarchy::ChildOf.
• document_order_map: entity -> PaintOrder, assigned by a pre-order DFS over the hierarchy forest. Parents rank before their children; siblings rank in Children-list order (bevy keeps the list in insertion order, which matches markup order for static spawns and reconcile order for runtime <if> / <for> clones). Entity-id allocation order plays no part, so entities spawned out of document order (children before parents, reconciler respawns) still paint in tree order and ExtractedClipBox ranges bracket exactly the descendant set.

Ranks are multiplied by 2 (see PaintOrder) so extract_shadows can place shadows at order - 1 without colliding with the preceding leaf. Consume the maps via paint_order_of.

Subtrees rooted at an OverlayLayer entity are excluded from the normal-band DFS and re-ranked into the top-layer band (>= OVERLAY_ORDER_BASE), stacked among themselves by OverlayOpenOrder stamp (later-opened on top), each keeping contiguous internal pre-order ranks. Idempotent within a frame: repeated calls (one per extract fn) see the same visibility state and hand out the same ranks.