Adds a single handoff document summarizing the mandatory rules (R1-R4,
zip-first), project overview, build/test/release commands, current state
(v0.3.9 released), the pending 'unify invalidate procedure' task with the
user-approved warning popup text, and a key file/line map. Lets any new chat
or AI model resume work without losing context. Documentation only; no plugin
code changed.
