Adds docs/refactor-roadmap.md, sequencing the design-debt issues from the design review into dependency-ordered phases.

What

• Orders issues Define a single source of truth for the codec format contract #23–Remove dead code and empty placeholder files; move selftest out of the CLI binary #30 against the existing hardening/format/perf issues (Bound decoder allocations before trusting LAC block tables #3, Make multithreaded encoder startup exception-safe #5, Reject non-canonical LAC and block metadata during decode #7, Eliminate undefined or implementation-defined arithmetic in codec hot paths #8, Add malformed-input regression tests for LAC and WAV parsers #11, Add fuzzing harnesses for WAV, LAC, block, Rice, and bitstream decoders #12, Reduce peak memory and repeated analysis in encode/decode pipelines #15).
• Phase 1 foundation/tests → Phase 2 core arithmetic → Phase 3 decode-path consolidation → Phase 4 structure/platform.
• Each phase has a completion Gate; the lossless round-trip and malformed-input rejection are stated as invariants held at every step.
• Documentation only — no code changes.

Notes

• Aligns with maintainer-workflow.md, which lists roadmap items as an expected maintenance artifact.
• Living document; intended to be updated as issues are closed or reordered.