No API to suppress Guardian boundary during walking/gait tasks (Unbounded mode not supported)

## Summary

During walking assessment tasks in our clinical app, the Guardian boundary wall fades in and obscures our navigation markers when the patient walks more than a few meters. We need to suppress or expand the boundary programmatically during supervised walking tasks.

## What We Observe

- `XRInputSubsystem.GetSupportedTrackingOriginModes()` returns `0x3` (Floor + Device only) — **Unbounded is not supported**
- `TrySetTrackingOriginMode(TrackingOriginModeFlags.Unbounded)` is not available
- `TrySetBoundaryPoints(empty list)` via reflection — method exists but returns `false`
- `TryRecenter()` works but only delays boundary appearance; does not prevent it
- There is no visible API to toggle boundary wall visibility

## What We Have Tried

1. Setting tracking origin to Unbounded — not in supported modes
2. Clearing boundary points via `TrySetBoundaryPoints(new List())` — returns false
3. Switching to Device tracking mode — boundary still appears
4. Recentering the play space — delays but does not prevent boundary
5. Rendering our gait markers with `ZTest Always` to show through boundary fade — partially works but poor UX

## Use Case

SDNE is a clinical neurologic exam that includes standard gait assessment tasks:
- **10-Meter Walk Test** — patient walks 3-10 meters at normal pace (speed, cadence, stride length measurement)
- **Timed Up-and-Go** — patient stands, walks 3 meters, turns, walks back, sits (fall risk assessment)

These tasks are performed under medical assistant supervision with passthrough enabled. The boundary wall interfering with walking guidance is a significant clinical UX problem — patients may stop walking or become disoriented when the boundary fades in.

## Questions

1. Is there an API or intent to temporarily suppress the Guardian boundary for supervised activities?
2. Is `XR_ANDROID_unbounded_reference_space` planned for Galaxy XR in a future firmware/SDK update?
3. Is there a way to programmatically expand the boundary to a larger area from within the app?

## Environment

- Device: Samsung Galaxy XR (SM_I610)
- OS: Android XR
- Unity: 6000.3.6f1
- OpenXR runtime
- Tracking origin: Floor mode (0x1)
- Passthrough enabled during gait tasks

No repository files, tests, or entry points are identified in the issue. Start by reviewing the reported tracking-origin and boundary API behavior on the stated Galaxy XR and Unity environment; done means establishing whether supervised suppression, expansion, or a future unbounded mode is supported and documenting the resulting API or limitation.

Written by the indexing model from the issue text.