Add adaptive XR quality control

[reality2-roycdavies]

What changed

• replace movement-only viewport scaling with a testable adaptive XR quality controller
• preserve the existing 0.65 walking and 0.55 sprinting profiles
• detect headset rotation and use a 0.78 motion profile while turning in place
• lower scale only after repeated missed frames and recover slowly after a stable delay
• honour runtime-recommended WebXR viewport scale values when available
• avoid repeated viewport requests for insignificant scale changes

Why

The previous controller treated a stationary player as fully static even during rapid head turns and did not react to sustained missed frames. That can apply maximum resolution during the most latency-sensitive motion while leaving no feedback mechanism for unexpectedly expensive scenes.

User impact

Static scenes retain full resolution. Locomotion retains the already-tested quality profiles. Head turning and sustained frame misses now receive temporary performance headroom, with hysteresis intended to prevent visible quality pumping.

Validation

• npm test: 210 tests passed, including 7 adaptive-controller tests
• modified JavaScript passes node --check
• git diff --check passes
• the browser resolved and parsed the complete v17 module graph before the container-level WebGL2 restriction