Phase 540: whiteboard persistence and render-layer readiness gate

[ExplodingUFO]

Context

Phase 539 is tracked by GitHub #201 / avalonia-node-map-rwr and PR #202. It records the rectangle/freehand drawing primitive model gate, but the next roadmap row remains Phase 540: whiteboard persistence and render-layer readiness gate.

React Flow whiteboard parity cannot be claimed from selection rectangles, lasso snapshots, or model-gate documentation alone. AsterGraph still needs a clear decision record for whether future rectangle/freehand primitives require persistence/schema support, renderer projection, hit-testing hooks, screenshot policy, and migration boundaries before any implementation starts.

Scope

Create a bounded docs/tests readiness gate for future whiteboard persistence and render-layer work:

• inventory existing persistence/schema, renderer projection, screenshot manifest, and Cookbook evidence relevant to future drawing primitives;
• separate current graph-scene persistence/rendering from whiteboard annotation persistence/rendering;
• define minimum readiness criteria before any rectangle/freehand drawing state can be persisted or rendered as first-class whiteboard primitives;
• update EN/ZH React Flow parity roadmap docs with this GitHub issue and Beads identifiers;
• replace the Phase 540 TBD row while preserving implementation work as future tracker-backed issues;
• add focused docs tests that fail if the roadmap implies whiteboard persistence/render-layer parity is already supported.

Out of scope

No runtime behavior changes, no public API changes, no persistence/schema changes, no renderer-layer changes, no screenshot manifest expansion, no Avalonia pointer coordinator edits, no UI redesign, no drawing tool implementation, no eraser behavior, no toolbar work, no retained API removal, and no full React Flow whiteboard parity claim.

Dependency / stacking

This issue depends on Phase 539 / PR #202. It may be implemented as a stacked docs/test branch based on feature/phase-539-drawing-primitive-model-gate while PR #202 is awaiting review, but it must not merge before Phase 539.

Acceptance Criteria

• EN/ZH parity docs record Phase 540 with this GitHub issue and the matching Beads ID.
• Docs explicitly separate current graph-scene persistence/rendering evidence from future whiteboard annotation persistence/rendering.
• Docs define minimum future readiness criteria, including persistence/schema contract, renderer projection, hit-testing/edit lifecycle, screenshot/Cookbook evidence, migration policy, and non-goals.
• Focused docs tests guard the Phase 540 boundary in both locales and fail if docs imply whiteboard persistence/render-layer parity is already supported.
• Next Issue Wave replaces the Phase 540 TBD row while leaving implementation as later tracker-backed work.
• Verification includes focused docs tests, full ReactFlowParityRoadmapDocsTests, full AsterGraph.Demo.Tests, and git diff --check.