Slate v2 ProseMirror + Lexical Roadmap Scan

Supporting plan. For current queue and roadmap truth, see master-roadmap.md.

Goal

Run a fresh focused code scan of:

• /Users/zbeyens/git/prosemirror
• /Users/zbeyens/git/lexical

for these seams:

• transaction/update model
• history/bookmark semantics
• clipboard boundary
• extension/plugin composition
• React/runtime subscription model

Then revise package-end-state-roadmap.md with package-level borrow / reject / later guidance.

Phases

1. Map exact source files in ../prosemirror for the five seams.
2. Map exact source files in ../lexical for the five seams.
3. Extract concrete takeaways for:
   • slate
   • slate-dom
   • slate-react
   • slate-history
   • slate-browser
4. Revise the roadmap doc with an explicit appendix:
   • borrow now
   • reject
   • later
5. Verify formatting and summarize what changed.