* Fix StyleBoxTexture using negative bounds UIBox2
Admin menu calls first draw with 0,0 bounds so the margin makes it negative; we just clamp it.
* a test fix
* Cache texture UVs
O(0) beats doing basic maths operations. Followup work to make some hot drawing paths (fonts + RSIs) use the AtlasTexture methods instead.
* 2026-08-02: Support headless atlas textures
Robust.UnitTesting was both ALL tests for RT, and also API surface for content tests.
Tests are now split into separate projects as appropriate, and the API side has also been split off.
