ci: add Codeberg CI (Woodpecker + Forgejo) and document CI/CD topology

Add a lightweight test-fast pipeline for the public Codeberg mirror so it
shows a build status, defined for both CI systems Codeberg offers:
- .woodpecker.yml            (Woodpecker, debian:bookworm)
- .forgejo/workflows/        (Forgejo Actions, node:20-bookworm, runner label docker)

Both run the pure-math suite only (Eigen vendored, GTest via FetchContent;
no CGAL/Boost), using public images and the docker runner label since the
private eulernest ci-cpp image and Pi runner are unreachable from Codeberg.

Docs:
- new doc/architecture/ci-cd.md: two-forge topology, runners, trigger
  keywords, mirror mechanism.
- fix stale "codeberg/main ... no CI" claim in contributing.md and bring
  the CI table up to date.
- link the new doc from the README doc index.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -142,6 +142,7 @@ Layout2D layout = euclidean_layout(mesh, res.x, maps);
 | **geometry-central comparison** — shared core, demarcation, adoption candidates, scientific added value | [doc/architecture/geometry-central-comparison.md](doc/architecture/geometry-central-comparison.md) |
 | **Design decisions** — key architectural choices + rationale | [doc/architecture/design-decisions.md](doc/architecture/design-decisions.md) |
 | **Project structure** — directory tree + build targets | [doc/architecture/project-structure.md](doc/architecture/project-structure.md) |
+| **CI / CD** — two-forge topology, runners, trigger keywords, mirror | [doc/architecture/ci-cd.md](doc/architecture/ci-cd.md) |
 | **Discrete conformal theory** — mathematical background for collaborators | [doc/math/discrete-conformal-theory.md](doc/math/discrete-conformal-theory.md) |
 | **Validation** — known analytic results + how to verify them | [doc/math/validation.md](doc/math/validation.md) |
 | **Validation protocol** — concrete commands with expected outputs | [doc/math/validation-protocol.md](doc/math/validation-protocol.md) |