feat: support AprilTag 36h11 for ARENA XR scene relocalization

[Copilot]

ARENA uses tag36h11 AprilTags (tag #0 = scene origin) for physical relocalization. The commonly referenced jp.keijiro.apriltag package only ships tagStandard41h12 native binaries, making it unusable for ARENA without recompilation. This PR bundles a self-contained native plugin compiled from AprilRobotics/apriltag source with 36h11 enabled, plus a new ArenaAprilTag MonoBehaviour for XR relocalization.

Native plugin (Runtime/AprilTag/Plugin/)

• libArenaAprilTag.so (Linux x86-64) — compiled from upstream apriltag source, exports tag36h11_create/destroy alongside the full detector API
• Named ArenaAprilTag (not AprilTag) to avoid conflicts if jp.keijiro.apriltag is also installed
• Plugin/README.md — cmake build instructions for Windows, macOS, iOS, Android

C# apriltag layer (Runtime/AprilTag/)

Self-contained P/Invoke layer adapted from the keijiro package (BSD-2-Clause attributed), with two key changes:

• Interop/Family.cs — wires tag36h11_create/destroy instead of tagStandard41h12
• Unity/TagDetector.cs — detector configured for tag36h11; uses Unity Jobs + Burst for pose estimation

ArenaAprilTag MonoBehaviour (Runtime/ArenaAprilTag.cs)

// Add to XR rig or main camera. No additional setup required when
// ArenaClientScene is present — sceneRoot defaults to its transform.
[DisallowMultipleComponent]
public class ArenaAprilTag : MonoBehaviour
{
    public float tagSize = 0.15f;       // physical tag size in meters
    public int   originTagId = 0;       // ARENA origin tag (default: 0)
    public int   decimation = 2;        // speed/accuracy trade-off
    public bool  smoothing = true;      // EMA smoothing across frames
    public float smoothingFactor = 0.2f;
    public Transform sceneRoot;         // falls back to ArenaClientScene.Instance
}

Each frame: captures WebCamTexture, detects 36h11 tags, and when tag originTagId is seen, repositions sceneRoot so the physical tag maps to (0,0,0) / identity rotation in Unity world space (with optional EMA smoothing). Pose math uses the tag world-space inversion:

newRootRot = Inverse(tagWorldRot)
newRootPos = -(Inverse(tagWorldRot) * tagWorldPos)

Other

• .gitattributes — marks .so/.dll/.a/.bundle as binary
• Third Party Notices.md — adds AprilRobotics/apriltag and keijiro attributions