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.

docs/TODO-1.0.md

@@ -1,5 +1,14 @@
 # TODO-1.0
 
+> **`docs/PLAN.md` is now the roadmap.** All seventeen items below have been
+> consolidated into areas and sequenced into the Phase → Wave → Track hierarchy in
+> `docs/PLAN.md`; that file is what implementation specialists are dispatched against,
+> and each of its tracks is self-sufficient for a brief. **This file is retained as the
+> verbatim-provenance spec appendix** — Daniel's raw asks and every answer round,
+> unedited, are the source of truth behind PLAN.md's compressed behavior bullets. Its
+> traceability table maps each item number below onto the track that owns it. Nothing in
+> this file changes as work lands; PLAN.md points move to `docs/COMPLETED.md`.
+
 Post-1.0 queue for ReaSampler — chiefly the 9000 instrument, plus two
 extension-side bugs. Items 1–3 are the first batch, in Daniel's ordering
 (2026-07-28); items 4–13 are a second batch (2026-07-28, later the same day);