Route RTT region readbacks through backend

docs/modernization/build-inventory.md

@@ -290,7 +290,9 @@ Known local toolchain state:
   allocation/storage/delete and framebuffer depth attach/detach also execute
   through tested dispatch contracts here. Renderer API render-pass color/depth/stencil
   clear-mask and clear-value mapping, and color-write-mask query tokens. `RTT` no longer
-  spells GL enum names directly. It also
+  spells GL enum names directly. `RTT` also exposes a retained RGBA8
+  region-readback helper that uses the tested framebuffer readback dispatch for
+  canvas pick/history/snapshot and transform history paths. It also
   validates renderer API primitive-topology to OpenGL draw-mode mapping, Shape
   index-type, fill/stroke primitive-mode, buffer target, static upload usage,
   and vertex attribute component/normalization mapping used by
@@ -410,12 +412,14 @@ Known local toolchain state:
   formats for cube-strip imports. Clamp-to-border sampler wrap is now part of
   the backend capability catalog and test coverage.
   Early canvas draw helpers also consume backend-owned pick readback
-  format/type, stroke mixer depth/scissor/blend state, saved viewport and
-  clear-state queries, active texture units, fallback 2D texture unbind
-  targets, and stroke background copy targets.
+  format/type and RTT-backed region-readback execution, stroke mixer
+  depth/scissor/blend state, saved viewport and clear-state queries, active
+  texture units, fallback 2D texture unbind targets, and stroke background copy
+  targets.
   Canvas stroke commit also consumes backend-owned saved viewport/clear/blend
-  state, history readback format/type, active texture units, fallback 2D
-  texture unbind targets, and layer compositing copy targets.
+  state, history readback format/type and RTT-backed region-readback execution,
+  active texture units, fallback 2D texture unbind targets, and layer
+  compositing copy targets.
   Canvas layer merge rendering and explicit layer-merge compositing also consume
   backend-owned depth/blend state, active texture units, fallback 2D texture
   unbind targets, and merge framebuffer copy targets.
@@ -610,7 +614,8 @@ Known local toolchain state:
   VR draw state setup and restore, tested saved-state snapshot/restore dispatch
   consumed by the retained `gl_state` utility, tested texture lifecycle/readback dispatch consumed by
   the retained `Texture2D` utility, tested framebuffer blit/readback dispatch
-  consumed by retained `RTT` resize/copy/readback paths, tested framebuffer
+  consumed by retained `RTT` resize/copy/readback and RGBA8 region-readback
+  paths, tested framebuffer
   bind/restore dispatch consumed by retained `RTT` render-target pass entry
   and exit paths, tested depth renderbuffer allocation/delete and framebuffer
   depth attach/detach dispatch consumed by canvas object-drawing helpers,

docs/modernization/roadmap.md

@@ -810,6 +810,10 @@ resource lifecycle path behind the renderer backend boundary.
 Legacy `RTT` resize/copy blits and byte/float framebuffer readbacks now execute
 through tested `pp_renderer_gl` framebuffer dispatch contracts with draw/read
 framebuffer binding restore handled by the backend helper.
+Legacy `RTT` also exposes an RGBA8 region-readback helper that uses the same
+backend framebuffer readback dispatch; canvas pick/history/snapshot and
+transform history paths now call that helper instead of binding an RTT and
+calling `glReadPixels` directly.
 Legacy `RTT::bindFramebuffer` and `RTT::unbindFramebuffer` now use tested
 `pp_renderer_gl` draw/read framebuffer binding snapshot and restore contracts,
 moving render-target pass entry/exit state management behind the backend.
@@ -1975,8 +1979,9 @@ Results:
   and fallback 2D texture unbinds through the renderer GL backend mapping;
   platform VR SDK bridges remain isolated for later platform-shell extraction.
 - Canvas mode overlay, mask, and transform paths now route generic OpenGL
-  blend/depth state, active texture units, 2D copy targets, and RGBA8
-  readback formats through the renderer GL backend mapping.
+  blend/depth state, active texture units, 2D copy targets, RGBA8 readback
+  formats, and RTT-backed transform history region readbacks through the
+  renderer GL backend mapping.
 - `NodeCanvas` panorama UI rendering now routes sampler defaults, saved
   viewport/clear/blend/depth/scissor state, color clears, active texture units,
   fallback 2D texture unbinds, copy targets, and RGBA8 render-target formats
@@ -1992,7 +1997,9 @@ Results:
   renderer GL backend mapping.
 - Canvas stroke commit now routes saved viewport/clear/blend state, history
   readbacks, active texture units, fallback 2D texture unbinds, and layer
-  compositing copy targets through the renderer GL backend mapping.
+  compositing copy targets through the renderer GL backend mapping; the
+  RTT-backed dirty-region readbacks now execute through the retained `RTT`
+  region-readback helper rather than direct `glReadPixels`.
 - Canvas layer merge rendering and explicit layer-merge compositing now route
   depth/blend state, active texture units, fallback 2D texture unbinds, and
   merge framebuffer copy targets through the renderer GL backend mapping.