Reject duplicate document snapshot payloads

docs/modernization/build-inventory.md

@@ -96,7 +96,7 @@ Known local toolchain state:
   including foundation binary-stream/event/logging/task queue coverage, PNG metadata and
   decode, PPI header/layout/non-finite opacity and blend-mode rejection, settings document, document
   snapshot/per-layer-frame/move/duration/face-pixel/PPI export coverage,
-  snapshot-embedded face-payload rejection, paint brush/final-blend/
+  snapshot-embedded duplicate/invalid face-payload and selection-mask rejection, paint brush/final-blend/
   stroke-alpha-blend/stroke spacing/stroke stress/stroke-script coverage,
   renderer shader descriptor and OpenGL capability coverage, UI
   color parsing, and layout XML parse coverage.

docs/modernization/roadmap.md

@@ -330,7 +330,8 @@ construction, per-layer frame metadata, layer metadata operations, frame
 move/duration queries, renderer-free RGBA8 cube-face payload storage,
 renderer-free alpha8 selection-mask storage, PPI image import/export, and
 layer/frame/undo-redo history invariant tests. Snapshot construction validates
-embedded face-pixel payload bounds and byte counts.
+embedded face-pixel payload bounds, byte counts, duplicate face payloads, and
+duplicate selection masks.
 `pp_renderer_api` has started with renderer-neutral
 texture/readback descriptors and validation tests. `pp_paint_renderer` has
 started with deterministic CPU layer compositing over renderer extents using