docs: add PLAN.md — post-1.0 roadmap consolidating TODO-1.0's seventeen items
Two Greek-lettered phases (Θ instrument overhaul, Ξ resample loop): sequenced waves, parallel tracks on disjoint surfaces, per-track acceptance criteria, carried-forward open questions, traceability table. TODO-1.0.md retained as verbatim-provenance appendix.
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