Consolidate design docs: one current design per topic, archive superseded ones, cut ROADMAP to what is left #15

docs/ was only re-filed into folders (design/ 14 files ~550 KB, roadmap/ROADMAP.md 288 KB, investigations/ 5, platforms/ 5). Content still overlaps (three upcall documents, ~190 KB), and several documents describe designs that are now superseded: the binder no longer renames to snake_case or occupies pythonx (d00f413), stubs are Kotlin-names only, Compose is 1.11.1, free-threading is 3.14t desktop opt-in.

Completion:

• docs/design/: one current document per topic that agrees with docs/INTENT.md, docs/SPEC.md and the code; the three upcall docs merged into one.
• Superseded designs (pythonx-adapter-design, the Pythonic stub product in pyi-generation-design, the manifest in typedpython) moved to docs/archive/ with a superseded header; still-valid parts that belong to pythonx-compose listed for that repo.
• docs/roadmap/ROADMAP.md reduced to what is left to do; history stays in git.
• docs/investigations/: conclusion and numbers kept, narrative to archive.
• Every in-repo reference to a moved document updated; tools/check_guide.py and a markdown link scan pass.