Goal

Do one narrow non-React @platejs/markdown pass after the March 14 helper/fallback work.

Highest-Value Seams

• MarkdownPlugin
• deserializeMd
• convertNodesDeserialize
• serializeMd
• convertNodesSerialize

Scope

• direct plugin API and parser contract
• direct deserializer behavior for custom rule wrapping, memoized output, and onError + withoutMdx
• serializer passthrough for editor value and remarkStringifyOptions
• one deserializer edge lane for conflicting filters and unknown nodes
• one serializer edge lane for conflicting filters and withBlockId

Explicit Deferrals

• defaultRules deep sweep
• broader mdx/list/mention matrices already covered elsewhere
• /react

Verification Plan

• targeted bun test on touched markdown specs
• bun test packages/markdown/src
• pnpm test:profile -- --top 15 packages/markdown/src
• pnpm test:slowest -- --top 15 packages/markdown/src
• pnpm install
• pnpm turbo build --filter=./packages/markdown
• pnpm turbo typecheck --filter=./packages/markdown
• pnpm lint:fix

Result

• added direct MarkdownPlugin contract coverage for defaults, bound API, and parser deserialization
• added direct deserializeMd coverage for top-level text wrapping, memoized output, and onError + withoutMdx
• added serializer coverage for direct serializeMd output and remarkStringifyOptions
• added deserializer and serializer edge coverage for conflicting filters, unknown nodes, and withBlockId
• no runtime bug surfaced; this stayed test-only