feat: add look_at_entity field to Billboard component

[decentraland-bot]

Summary

• Adds an optional look_at_entity field (uint32) to the PBBillboard protobuf message
• When set, the Billboard rotates to face the world-space Transform position of the specified entity rather than the camera
• Fully backward-compatible: omitting the field preserves all existing camera-facing behavior

Behavior

Scenario	Result
look_at_entity not set	Billboard faces the camera (existing default behavior)
look_at_entity set to a valid entity with a Transform	Billboard faces that entity's world-space position
look_at_entity set to a non-existent or Transform-less entity	Billboard falls back to facing the camera

The billboard_mode field continues to control which axes (X/Y/Z) are affected by the rotation, regardless of whether look_at_entity is set.

Changes

• proto/decentraland/sdk/components/billboard.proto: Added optional uint32 look_at_entity = 2; with comprehensive inline documentation

Related PRs

• SDK: decentraland/js-sdk-toolchain — adds lookAtEntity to the TypeScript API surface and tests
• Explorer: decentraland/unity-explorer — implements the entity-lookup logic in BillboardSystem

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🤖 Created via Slack with Claude
Requested by Nico Earnshaw via Slack

[github-actions]

Test this pull request

• The @dcl/protocol package can be tested in scenes by running

  npm install "https://sdk-team-cdn.decentraland.org/@dcl/protocol/branch//dcl-protocol-1.0.0-24530609736.commit-d60a3e6.tgz"