docs: restructure documentation into focused files

README.md: reduced from 703 to ~75 lines — what/why, status, quick
start, minimal usage example, navigation table to doc/ files.

doc/architecture/overall_pipeline.md: trimmed — roadmap, extension
points, declarative pipeline YAML, and references sections removed
(each now has its own dedicated file). Replaced with a link table.

New files:
  doc/getting-started.md       — build modes, single-test invocation, CLI
  doc/api/pipeline.md          — full pipeline API with code for all 3 geometries
  doc/api/extending.md         — new functionals, geometry modes, Java porting guide
  doc/api/contracts.md         — processing unit preconditions/provides table
  doc/api/cgal-package.md      — Phase 8 CGAL package design + YAML pipeline (TODO)
  doc/math/geometry-modes.md   — Euclidean/Spherical/HyperIdeal comparison
  doc/math/references.md       — all papers by module
  doc/roadmap/phases.md        — Phases 1–10 with porting/research boundary
  doc/roadmap/java-parity.md   — Java vs C++ feature parity table
  doc/contributing.md          — language policy, test standards, release flow

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

doc/contributing.md

@@ -0,0 +1,88 @@
+# Contributing
+
+## Language
+
+**All code, comments, documentation, commit messages, and test descriptions must be in English.**
+
+The project targets CGAL submission and international collaboration. When editing
+files that still contain German-language comments or documentation, replace them
+with English.
+
+---
+
+## Git workflow
+
+- `main` is protected on `origin` (Gitea). Push to `dev`, then open a pull request.
+- `codeberg/main` can be pushed to directly (public mirror, no CI).
+- Both remotes must stay in sync after every significant change:
+  ```bash
+  git push origin HEAD:dev   # triggers CI
+  git push codeberg main     # updates public mirror
+  ```
+- Branch naming: `feature/<topic>`, `fix/<topic>`, `phase<N>-<topic>`
+
+---
+
+## CI
+
+Two jobs run on push to `dev`/`main` or on pull requests:
+
+| Job | What it tests | Trigger |
+|---|---|---|
+| `test-fast` | 36 non-CGAL tests, no Boost | all branches |
+| `test-cgal` | 158 CGAL tests + 2 skips | `main`, `dev`, PRs only |
+
+A PR is ready to merge when both jobs pass.
+
+The runner is a self-hosted Raspberry Pi (ARM64). See `.gitea/workflows/cpp-tests.yml`
+and `.gitea/docker/Dockerfile.ci-cpp`.
+
+---
+
+## Test standards
+
+Every new algorithm needs:
+
+1. **Gradient check** — finite-difference verification of `G(x) = ∂E/∂x`.
+   Copy any `GradientCheck_*` test suite from `tests/cgal/test_*_functional.cpp`.
+
+2. **Convergence test** — Newton converges on a small mesh using the "natural theta"
+   trick (see [CLAUDE.md](../CLAUDE.md) and [api/extending.md](api/extending.md)).
+
+3. **Registration** — add the `.cpp` file to `code/tests/cgal/CMakeLists.txt`.
+
+Expected CI result: **36 + 158 tests pass, exactly 2 skipped**.
+The 2 skips are intentional `GTEST_SKIP()` stubs for Phase 9b (analytic HyperIdeal Hessian)
+and Phase 9c (genus-2 homology). Do not remove them.
+
+---
+
+## Code style
+
+- C++17. Header-only (`code/include/*.hpp`). No compiled library.
+- `#pragma once` at the top of every header.
+- Everything in the `conformallab` namespace, internal helpers in `conformallab::detail`.
+- Property map names follow the prefix convention: `"v:"` (vertex), `"e:"` (edge), `"f:"` (face).
+- DOF index `-1` always means "pinned/fixed".
+
+---
+
+## Releases
+
+Release tags follow `vMAJOR.MINOR.PATCH`:
+
+```bash
+# Merge dev → main, then tag
+git checkout main && git merge --no-ff dev
+git tag -a vX.Y.Z -m "vX.Y.Z — <one-line summary>"
+git push origin main && git push origin vX.Y.Z
+git push codeberg main && git push codeberg vX.Y.Z
+```
+
+---
+
+## TODO
+
+- [ ] Define code review checklist
+- [ ] Add `.clang-format` configuration
+- [ ] Document how to request CGAL package review