Add project documentation and framework setup

- Initialize project documentation structure
- Add business case and stakeholder analysis
- Create project plan with four milestones
- Setup framework integration via git submodule
- Configure gitignore for project structure

docs/milestones/mil-001-project-scaffold.md

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+# MIL-001 Project scaffold
+
+## Metadata
+| Key | Value |
+| --- | --- |
+| ID | MIL-001 |
+| CrossReference | [PP-001], [SA-001], [BC-001] |
+
+## Version History
+| Date | Status | Author | Reviewer | Change | Commit |
+| --- | --- | --- | --- | --- | --- |
+| 2026-10-04 | Accepted | Jens Tirsvad Nielsen | S01 | Initial version | pending |
+
+---
+
+## Purpose
+
+Decide whether the repository is ready for code: layout, tooling and configuration exist and a fresh clone can create a local virtual environment.
+
+## Deliverable
+
+`pyproject.toml`, Python `.gitignore`, `src/calc/` and `tests/` skeletons, `Doxyfile`, repository description and topics on the git host.
+
+## Go / No-Go Criteria
+
+| # | Criterion (objectively checkable) | Go | No-Go |
+| --- | --- | --- | --- |
+| 1 | `pyproject.toml` requires Python 3.13 or newer and has no runtime dependencies | Met | Not met |
+| 2 | `.gitignore` covers `.venv`, `.env` and `docs/doxygen/` | Met | Not met |
+| 3 | Folders `src/`, `tests/` and `docs/` exist | Met | Not met |
+| 4 | Repository description and topics are set on the git host | Met | Not met |
+
+## Dependencies
+
+| Depends on | Reason |
+| --- | --- |
+| None | First milestone |
+
+## Traceability
+
+| Business Case objective / KPI / user story | Reference |
+| --- | --- |
+| Objectives 1 to 4 | [BC-001] |
+
+## Ownership
+
+| Role | Stakeholder ID (SA) |
+| --- | --- |
+| Owner | S01 |
+| Approving reviewer | S01 |
+
+## Target Date
+
+2026-10-05
+
+## Tasks
+
+| # | Task | Summary | Needs its own Use Case/User Story? | Reference |
+| --- | --- | --- | --- | --- |
+| 1 | Add pyproject.toml | Project configuration: name, version, Python 3.13+ requirement, empty runtime dependencies, `dev` extra with pytest, console script `calc`, pytest settings. | No |  |
+| 2 | Add Python .gitignore and folder layout | Python `.gitignore` that also ignores `.env`, `.venv` and `docs/doxygen/`; create `src/calc/` and `tests/`. | No |  |
+| 3 | Add Doxyfile | Doxygen configuration reading `src/`, writing to `docs/doxygen/`, so Doxygen-style comments can be built into API docs. | No |  |
+| 4 | Set repository description and topics | Set the description and topics on the git host repository through its API, with the token from `.env`, which is personal tooling and never part of the project. | No |  |
+
+---
+
+[PP-001]: ../project-plan.md
+[SA-001]: ../stakeholder-analysis.md
+[BC-001]: ../business-case.md