docs: fill information gaps — headers, tests, design decisions, project structure

doc/api/headers.md              — all 24 public headers with descriptions
doc/api/tests.md                — 26 test suites, individual counts, run instructions
doc/architecture/design-decisions.md  — 5 key design choices with rationale
doc/architecture/project-structure.md — full directory tree + build targets

README + overall_pipeline.md link tables updated.

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

doc/architecture/overall_pipeline.md

@@ -392,9 +392,13 @@ Normalise     PCA centring         Rodrigues to N pole  weighted Möbius centrin
 
 | Topic | Document |
 |---|---|
+| Public headers (all 24, with descriptions) | [api/headers.md](../api/headers.md) |
+| Test suites (26 suites, 158 tests, individual counts) | [api/tests.md](../api/tests.md) |
 | Extending the library (new functionals, geometry modes, Java porting guide) | [api/extending.md](../api/extending.md) |
 | Processing unit contracts (preconditions / provides table) | [api/contracts.md](../api/contracts.md) |
 | Phase 8 CGAL package design + declarative pipeline YAML | [api/cgal-package.md](../api/cgal-package.md) |
+| Key design decisions + rationale | [architecture/design-decisions.md](design-decisions.md) |
+| Project structure (directory tree + build targets) | [architecture/project-structure.md](project-structure.md) |
 | Three geometry modes in detail | [math/geometry-modes.md](../math/geometry-modes.md) |
 | References and papers | [math/references.md](../math/references.md) |
 | Development roadmap (Phases 1–10) | [roadmap/phases.md](../roadmap/phases.md) |