docs: classify post-stability model roadmap

[stefan-jansen]

Closes #27

Summary

• classify roadmap candidates across the five required library-fit areas
• distinguish accepted candidates from deferred proposals
• record explicit input/output contracts for each accepted candidate
• define required behavioral tests, installed-wheel checks, API reference, and worked examples

Verification

• uv run ruff check .
• uv run ruff format --check .
• uv run ty check
• uv run pytest
• uv run mkdocs build --strict
• uv build
• pre-commit run --all-files

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