Add renderer readback command contract

docs/modernization/build-inventory.md

@@ -285,10 +285,12 @@ Known local toolchain state:
   source code reintroduces raw `GL_*`/`WGL_*` constants outside the allowed
   legacy OpenGL implementation files.
 - `pp_renderer_api` exposes a headless `RecordingRenderDevice` that validates
-  command order, records render commands, and records trace markers without a
-  window or GL context.
+  command order, readback bounds, and destination buffer sizes, records render
+  and readback commands, and records trace markers without a window or GL
+  context.
 - `pano_cli record-render` exposes the recording renderer through JSON
-  automation and is covered by `pano_cli_record_render_smoke`.
+  automation, including readback command/byte totals, and is covered by
+  `pano_cli_record_render_smoke`.
 - `pano_cli simulate-document-history` exposes `pp_document::DocumentHistory`
   apply/undo/redo state through JSON automation and is covered by
   `pano_cli_simulate_document_history_smoke`.

docs/modernization/roadmap.md

@@ -416,8 +416,9 @@ adding new backends.
 Status: started. `pp_renderer_api` exists as a headless renderer-neutral target
 with texture descriptor, byte-size, viewport, mesh, readback bounds, command
 context, render device, shader program descriptor, mesh, render target,
-readback, trace interface validation, and the canonical PanoPainter shader
-catalog now consumed by the legacy OpenGL app initialization path.
+readback byte-size helpers, readback command validation, trace interface
+validation, and the canonical PanoPainter shader catalog now consumed by the
+legacy OpenGL app initialization path.
 `pp_renderer_gl` now exists as the first OpenGL backend library and owns pure
 OpenGL capability detection for framebuffer fetch, map-buffer alignment, and
 float texture support. It also owns the OpenGL texture upload-type mapping used
@@ -716,7 +717,9 @@ Results:
   per-layer frame duration, and PNG-encoded face-payload export to PPI bytes,
   plus malformed payload rejection at the export boundary.
 - `pp_renderer_api_tests` passed, including shader descriptor validation,
-  PanoPainter shader catalog validation, and invalid catalog rejection.
+  PanoPainter shader catalog validation, readback byte-size and command-order
+  validation, recording readback command capture, and invalid catalog
+  rejection.
 - `pp_paint_renderer_compositor_tests` passed.
 - `pp_ui_core_color_tests` passed.
 - `pp_ui_core_layout_value_tests` passed.
@@ -810,11 +813,12 @@ Results:
   reintroduces raw `GL_*`/`WGL_*` constants outside the allowed legacy OpenGL
   implementation files.
 - `pp_renderer_api` now includes a headless `RecordingRenderDevice` with strict
-  command-order validation and command/trace capture, giving automation a
-  backend-neutral render path that does not require a window or GL context.
+  command-order/readback validation and command/trace/readback capture, giving
+  automation a backend-neutral render path that does not require a window or GL
+  context.
 - `pano_cli record-render` exercises that headless recording renderer and emits
-  JSON command counts, target dimensions, backend name, and trace/draw summary
-  for agent automation.
+  JSON command counts, target dimensions, backend name, trace/draw summary, and
+  readback command/byte totals for agent automation.
 - `pano_cli simulate-document-history` exercises pure document history
   apply/undo/redo behavior and emits JSON layer/frame/history state for agent
   automation.