Adds XR_ANDROID_depth_texture as a native depth backend so Sensor.Depth and depth-based World.Occlusion work on Android XR behind the existing public API.

Unlike Meta's GPU swapchain (NDC d16), the Android extension delivers CPU float arrays in metric meters. A new sensor_depth_format_ field (ndc_d16 vs meters_r32) lets each backend expose its native format.

• CPU-direct vs GPU readback. Meta serves a GPU texture through the readback; Android serves CPU buffers directly. sensor_depth_try_get_latest_data is now provider-aware, and the C# TryGetLatestData has a texture-less path.
• Occlusion. Added a depth_format-gated path that reconstructs per-eye from fov tangents + pose.

Public API changes

• Permission: permission_type_scene_fine (SCENE_UNDERSTANDING_FINE)
• Enums: sensor_depth_format_ (ndc_d16/meters_r32), sensor_depth_image_; new sensor_depth_caps_ flags (raw / smooth / confidence)
• sensor_depth_frame_t fields: depth_format, view_count, available_images
• Signature changes: sensor_depth_start gains a resolution; sensor_depth_try_get_latest_data gains an image selector
• New: sensor_depth_get_resolutions
• C# mirrors all of the above (SensorDepthFormat/Image, caps flags, Sensor.Depth.Start(resolution), GetResolutions(), TryGetLatestData(..., image))

Testing

• Meta impl has no regressions
• XREAL Aura (raw + smooth, confidence coloring, and depth occlusion)
• Galaxy XR (same as above, can verify tomorrow)