docs: merge TODO into ROADMAP, delete TODO.org

- ROADMAP.org now has per-version ** Tasks sections with TODO/DONE states

- The TODO headings are the authoritative task tracker

- Feature tables kept for readability; checkboxes removed (state is in headlines)

- docs/TODO.org deleted

- All references updated to point to ROADMAP.org

README.org

@@ -59,7 +59,7 @@ See [[file:docs/USER_MANUAL.org][User Manual]] for the full guide.
 | [[file:docs/ARCHITECTURE.org][Architecture]] | How does it work inside? |
 | [[file:docs/DESIGN_DECISIONS.org][Design Decisions]] | Why was it built this way? |
 | [[file:docs/ROADMAP.org][Roadmap]] | Where is it going? When? |
-| [[file:docs/TODO.org][TODO]] | Who is doing what? |
+| [[file:docs/ROADMAP.org][TODO]] | Who is doing what? |
 | [[file:docs/CONTRIBUTING.org][Contributing]] | How do I contribute? |
 
 * License