Add billboard support for child label meshes

Problem

Babylon-Lite has atlas billboard systems, but a regular Mesh has no billboardMode or small helper equivalent to Babylon.js Mesh.BILLBOARDMODE_ALL. A dynamic-texture plane parented to a moving piece therefore inherits the piece rotation and cannot remain camera-facing without application-owned per-frame orientation math.

This is especially visible for counter values: a flat top label becomes unreadable at shallow orbit angles, while a camera-facing label remains useful throughout the camera range.

Minimal reproduction

import { createPlane, onBeforeRender, setParent } from "@babylonjs/lite";

const label = createPlane(engine, { width: 0.56, height: 0.33 });
setParent(label, counterMesh);
label.position.set(0, 0.68, 0);

// Missing: label.billboardMode = BILLBOARDMODE_ALL, or setBillboard(label, camera).
// The application must rebuild the local rotation quaternion every frame (pitch/yaw toward the camera, parent rotation removed).
onBeforeRender(scene, () => orientLabelByHand(label, counterMesh, camera));

Proposed API

A tree-shakeable helper that works for both root and child meshes would fit Lite’s functional API:

const removeBillboard = setBillboard(label, camera, {
  mode: "all", // or "yaw"
  compensateParent: true,
});
removeBillboard();

Alternatively, mesh.billboardMode with BILLBOARDMODE_NONE, BILLBOARDMODE_Y, and BILLBOARDMODE_ALL constants would ease Babylon.js migration. Parent rotation/scaling should be accounted for, the plane should stay upright in yaw mode, and cleanup must remove any per-frame hook.

Alternatives considered

• Keeping labels flat is cheap but loses readability at ordinary orbit angles.
• Reparenting labels to the scene avoids inherited rotation but requires manually copying world position and lifecycle from the piece.
• Application-local Euler/quaternion math is small, but is duplicated by every UI-label consumer and easy to get wrong for parent transforms.

Current downstream workaround

Digipology creates labels as child planes in addLabel. Counters are now raised and reoriented from the adapter’s existing per-frame callback (orientBillboardLabel: a pitch/yaw quaternion toward the camera with the parent piece rotation removed); cards and dice intentionally remain flat. That adapter-local code can be removed when a native child-mesh billboard helper is available.