Route paint texture unit binding through GL backend

docs/modernization/build-inventory.md

@@ -381,7 +381,10 @@ Known local toolchain state:
   texture binding targets, and active texture units.
   `NodeStrokePreview` brush preview rendering also consumes backend-owned
   depth/scissor/blend state, viewport/clear-color queries, active texture
-  units, 2D texture targets, copy targets, and sampler filters/wraps.
+  unit execution, fallback 2D texture unbinds, 2D texture targets, copy
+  targets, and sampler filters/wraps. Retained `Canvas` stroke/thumbnail paths
+  and `NodeCanvas` panorama rendering use the same tested active-texture
+  dispatch for texture-unit switches.
   Legacy `Texture2D`, `TextureManager`, `Sampler`, and `RTT` public headers no
   longer expose raw OpenGL enum defaults; default texture formats, sampler
   filters/wraps, and render-target formats resolve through backend-owned
@@ -633,6 +636,8 @@ Known local toolchain state:
   color/depth attachment, status-check, and binding-restore dispatch consumed
   by retained `RTT::create`/`RTT::destroy`, tested RTT render-target clear,
   masked color clear with color-write-mask restore, and texture-bind dispatch,
+  tested active-texture dispatch consumed by retained Canvas, NodeCanvas, and
+  NodeStrokePreview texture-unit switches,
   tested pixel-buffer allocation/readback/map/unmap/delete dispatch
   consumed by retained `PBO` recording readbacks, tested framebuffer-to-texture
   2D copy dispatch consumed by retained canvas/UI paint paths, tested framebuffer