fix(editor): guard stale whiteboard block writes

docs/ABSTRACTIONS.md

@@ -569,6 +569,7 @@ Defined in `src/utils/durableMarkdownBlocks.ts`, `src/utils/editorDurableMarkdow
 - Fenced `tldraw` blocks become `tldrawBlock` schema nodes before BlockNote sees the Markdown body.
 - Each `tldrawBlock` stores a stable `boardId` plus the tldraw document snapshot JSON. Session state such as camera, selected tool, and current selection is not persisted into the note.
 - The rich editor renders the block with the `tldraw` package and saves debounced document snapshot changes back into the block props, so normal Tolaria autosave writes the board into the `.md` file.
+- Whiteboard prop writes re-resolve the live BlockNote block by id before mutating it, and disappear as no-ops if a note reload or mode switch has already removed that block.
 - Mermaid and tldraw both register small codecs with the shared durable fenced-block pipeline; scanner, token, block injection, and mixed serialization mechanics live in one owner.
 - The `/whiteboard` slash command inserts an empty tldraw block using the same Markdown-durable storage path. Preview images are intentionally omitted; thumbnails can be added later as derived cache artifacts.