Document the already-decided decomposition of subplan-02 Slice 1 so the
P2.1 boundary is verifiable from the repo:
- P2.1 = Core Structs + boundary validation (validation portion of Slice 1)
- P2.3 = in-memory repository + db.json (rest of Slice 1)
- Token = Slice 5 (Token Security), out of both
Planning-doc record only; no domain or test code changed.
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