Define Phase 21 roadmap #205

PLAN.md currently ends at Phase 20, and there are no open issues or pull requests to drive the next implementation iteration. Define the next phase before creating further implementation work.\n\nAcceptance criteria:\n- Add a Phase 21 section to PLAN.md with a clear goal and concrete scope.\n- Keep the scope aligned with the project constraints in CLAUDE.md: zero external crates, pure Rust subsystems, and unsafe only in the allowed crates.\n- Include a test milestone for the phase and identify which existing test suites should be extended.\n- Break the phase into enough bullet points that the next automation pass can create self-contained Tangled issues in dependency order.\n- Do not implement Phase 21 features in this issue; this is roadmap definition only.