[G-4-R-Y]

What changed

• adds a small, shell-free external adapter for the Apache-2.0 Brush Gaussian-splat viewer
• adds experiments/22_brush_splat_viewer.py with dry-run support and explicit missing-binary behavior
• adds unit coverage for command construction/input validation
• records the 2026-09-15..22 research sweep, including Brush GPU/fusion work, SuperSplat 3.3.1's experimental viewport-warp path, Spark maintenance, and explicitly deferred/out-of-window references

Scope / safety

Brush stays out-of-process and opt-in. This PR does not download/install upstream software or alter core dependencies. It does not claim WebGPU, projector, fullscreen, or performance validation without target hardware.

Validation

Static/unit test coverage is included. Hardware-dependent behavior remains pending actual workstation/projector testing.