perf(render): speed up DataTree rendering and harden visual regression coverage

- add deterministic DataTree benchmark scenarios, baselines, and timing/regression tests
- optimize layout and paint paths with culling, guards, binary search, and cache cleanup
- fix snapshot capture, compositing, separator rendering, and font/color cache lifetime issues
- relayout on DPI changes and invalidate the watermark after structural updates
- remove obsolete hwnd diagnostics/tests and clean up supporting docs and agent metadata

Author: johnml1135

.GitHub/skills/render-testing/SKILL.md

@@ -10,6 +10,11 @@ This file documents the **supported** build workflow for FieldWorks.
 
 FieldWorks is **Windows-first** and **x64-only**. Use the repo scripts so build ordering (native before managed) is correct.
 
+## Non-Windows hosts
+- Linux and macOS are supported for editing, code search, documentation, specs, and agent workflows only.
+- Do not attempt to build, test, or run installer/setup flows on non-Windows hosts.
+- `build.ps1` and `test.ps1` intentionally fail fast on non-Windows hosts so automation notices the environment mismatch.
+
 ## Quick start (PowerShell)
 ```powershell
 # Full traversal build (Debug/x64 defaults)