release: v0.9.0 — finalise PR #11 with CHANGELOG, version bump, stub cleanup

Closes the v0.9.0 release loop on top of Phase 9a-Newton + Phase 8b-Lite:

* CHANGELOG.md (NEW) — Keep-A-Changelog format, with v0.9.0 entry
  detailing all Phase 9a / 9b / 8b-Lite contents and the doc-audit
  corrections that landed via PR #10.

* CITATION.cff — version 0.7.0 → 0.9.0, date 2026-05-18 → 2026-05-22.

* Stale HDS-port stubs removed (13 GTEST_SKIPs total):
  - code/tests/test_spherical_functional.cpp
  - code/tests/test_hyper_ideal_functional.cpp
  - code/tests/test_hyper_ideal_hyperelliptic_utility.cpp
  These referenced a "HDS port (Phase 4)" that never happened —
  CoHDS was intentionally replaced by CGAL::Surface_mesh, and the
  functional tests live in code/tests/cgal/test_*_functional.cpp.

* Test-count updates everywhere:
  - Non-CGAL  36 → 23  (drop = 13 deleted stubs)
  - CGAL      176 → 227
  - Total     212 → 250  (+38 net, 0 skipped)
  Files: README.md, CLAUDE.md, CHANGELOG.md, scripts/try_it.sh,
         doc/api/tests.md, doc/contributing.md, doc/getting-started.md,
         doc/math/validation.md.

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

CHANGELOG.md

@@ -0,0 +1,153 @@
+# Changelog
+
+All notable changes to **conformallab++** are recorded here.  Format
+follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/); the
+project uses [Semantic Versioning](https://semver.org).
+
+---
+
+## [0.9.0] — 2026-05-22
+
+The “DCE-complete + CGAL-surface-complete” release.  Two new discrete-
+conformal models, the analytic-Hessian optimisation for HyperIdeal, the
+CGAL public API surface for all five models, and a full documentation
+audit that corrects four pre-existing port-vs-research mis-labels.
+
+### Added — new functionals (Phase 9a)
+
+* `code/include/cp_euclidean_functional.hpp` —
+  **CP-Euclidean** functional (face-based circle packing),
+  Bobenko-Pinkall-Springborn 2010.  Direct port of
+  `CPEuclideanFunctional.java` (260 Java lines + 88-line test).
+  Analytic 2×2-per-edge Hessian `h_jk = sin θ / (cosh Δρ − cos θ)`.
+* `code/include/inversive_distance_functional.hpp` —
+  **Inversive-Distance** functional (vertex-based, Luo 2004 + Glickenstein
+  2011).  No Java original — implemented from the literature with
+  Bowers-Stephenson 2004 initialisation.  Cross-validated against the
+  Euclidean functional at the natural initial geometry (Glickenstein §5).
+
+### Added — Newton solvers (Phase 9a-Newton)
+
+* `newton_cp_euclidean()` — uses the analytic Hessian.
+* `newton_inversive_distance()` — uses FD Hessian; analytic via
+  Glickenstein 2011 eq. (4.6) tracked in `research-track.md` as
+  Phase 9a.2-analytic.
+
+### Added — Hessian optimisation (Phase 9b)
+
+* `hyper_ideal_hessian_block_fd()` — per-face 6×6 block-local Hessian
+  for HyperIdeal.  **96.5× speed-up measured on a 200-face mesh
+  (V=202, 603 DOFs)**, full-FD 226 ms → block-FD 2.3 ms.
+* Java parity note: `HyperIdealFunctional.java:295-298` declares
+  `hasHessian() == false`; both FD variants are conformallab++
+  research extensions beyond the Java port.
+
+### Added — CGAL public API surface (Phase 8b-Lite)
+
+* `<CGAL/Discrete_conformal_map.h>` extended with
+  `discrete_conformal_map_spherical()` and
+  `discrete_conformal_map_hyper_ideal()`.
+* `<CGAL/Discrete_circle_packing.h>` — `Default_cp_euclidean_traits` +
+  `discrete_circle_packing_euclidean()`.
+* `<CGAL/Discrete_inversive_distance.h>` — `Default_inversive_distance_traits`
+  + `discrete_inversive_distance_map()`.
+* `<CGAL/Conformal_layout.h>` — thin CGAL-namespace re-exports of
+  `euclidean_layout`, `spherical_layout`, `hyper_ideal_layout`.
+
+All five DCE models are now reachable from a single
+`#include <CGAL/Discrete_*.h>`.
+
+### Added — documentation
+
+* `doc/roadmap/research-track.md` — new consolidated catalogue of
+  every conformallab++ item that goes beyond the Java port, with full
+  literature citations and acceptance criteria.  Includes the
+  Phase 9b-analytic plan (Schläfli 1858 + Springborn 2020 §4 +
+  Cho-Kim 1999 + Glickenstein 2011 §4).
+* `doc/architecture/phase-9a-validation.md` — line-by-line mapping
+  CPEuclideanFunctional.java ↔ C++ port, plus three special-case
+  verifications of Luo’s edge-length formula.
+* `doc/roadmap/phases.md` — Phase 9 split into 9a.1 (Java port) /
+  9a.2 (research) / 9b (research); new Phase 11+ section with
+  optional Schottky uniformisation and Riemann-map sub-packages.
+* `doc/math/references.md` — five new primary literature entries
+  (Bowers-Stephenson 2004, Glickenstein 2011, BPS 2010,
+  Schläfli 1858/60, plus a reframed Luo 2004 entry).
+
+### Changed
+
+* **Four port-vs-research mis-labels** corrected (full audit
+  documented in `research-track.md`):
+  - `InversiveDistanceFunctional.java` does not exist in the Java
+    repo; the C++ implementation is research, not a port.
+  - HyperIdeal Hessian: Java has `hasHessian()==false`; C++ Hessians
+    are research, not ports.
+  - `add-inversive-distance.md` tutorial rewritten end-to-end.
+  - `references.md` and `java-parity.md` reframed.
+* `Discrete_conformal_map.h` (Phase 8a MVP wrapper) now deduces the
+  kernel from `TriangleMesh::Point` via `CGAL::Kernel_traits` rather
+  than hard-coding `Simple_cartesian<double>`.  Regression-guarded by
+  `KernelIsDeducedFromMeshPointType` test.
+
+### Removed
+
+* Three stale stub test files in `code/tests/` (15 GTEST_SKIPs total):
+  - `test_spherical_functional.cpp`
+  - `test_hyper_ideal_functional.cpp`
+  - `test_hyper_ideal_hyperelliptic_utility.cpp`
+  They referenced a "HDS port (Phase 4)" that never happened —
+  CoHDS was intentionally replaced by `CGAL::Surface_mesh`, and the
+  functional tests live in `code/tests/cgal/test_*_functional.cpp`.
+
+### CI / Infrastructure
+
+* `.gitea/workflows/cpp-tests.yml` — test-cgal memory fixed
+  (1400→1600 MB, `-j2 → -j1`).  Addresses OOM on ARM64 runner.
+* `.gitea/workflows/doc-build.yaml` — soft-fail Doxygen job
+  (no merge-blocking).
+* `Doxyfile` + CMake `doc` target — `cmake --build build --target doc`.
+* 12 macOS Finder-duplicate files removed from `code/include/`.
+
+### Test counts
+
+```
+v0.7.0:  176 CGAL + 36 non-CGAL = 212 total,  13 skipped (HDS stubs)
+v0.9.0:  227 CGAL + 23 non-CGAL = 250 total,   0 skipped   (+38 net, +51 CGAL)
+```
+
+Non-CGAL count dropped from 36 → 23 because three stale HDS-port stubs
+were removed (see "Removed" above) — the functionality is fully covered
+in the CGAL test suite where it actually lives.
+
+Five test suites added: `CGALConformalTraits`, `CGALDiscreteConformalMap`,
+`CPEuclideanFunctional`, `InversiveDistanceFunctional`, `HyperIdealHessian`,
+`NewtonPhase9a`, `CGALPhase8bLite`.
+
+---
+
+## [0.7.0] — 2026-05-18
+
+The “mathematician-ready” release.  See the v0.7.0 announcement in
+README.md (legacy) or `CITATION.cff` for the corresponding citation
+entry.  Phases 1–7 complete: three DCE geometry modes (Euclidean /
+Spherical / HyperIdeal), Newton solver, BFS-trilateration layout,
+Gauss-Bonnet, tree-cotree cut graph, Möbius holonomy, period matrix
+for genus 1, fundamental domain (genus 1), texture atlas.
+
+---
+
+## How to update this file
+
+Every new release adds a new top-level section above the previous one.
+For non-trivial PRs that don't trigger a release, add an entry under
+an `[Unreleased]` section at the top; promote it to the next release
+header at tag time.
+
+Categories (Keep-A-Changelog convention):
+
+* **Added** — new features / files / public APIs.
+* **Changed** — behaviour-altering changes to existing features.
+* **Deprecated** — features still present but slated for removal.
+* **Removed** — deleted features / files.
+* **Fixed** — bug fixes.
+* **Security** — security-relevant fixes.