doc(roadmap): add research item for 2/3-ideal hyper-ideal volume formulas

Follow-up to the Finding-A fix (face_energy guard).  The throw is the
correct safe behaviour for now; the mathematically complete solution
requires formulas for semi-ideal and fully-ideal tetrahedra that are
absent from the Java reference.

Adds a structured research entry to research-track.md (Phase 9b+):
- primary references: Kolpakov-Mednykh arXiv:math/0603097, Milnor 1982,
  Vinberg 1985
- acceptance criteria: two new volume functions + gradient-check tests +
  limiting-behaviour continuity witness
- explicit note that the throw in face_energy() must not be removed
  without implementing and testing the replacement formulas

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

doc/reviewer/external-audit-2026-05-30.md

@@ -170,6 +170,12 @@ Instead it:
 **Test result:** 262/262 CGAL tests pass (was 246 before + 3 new guard tests + 13 from
 prior suite growth). No regressions.
 
+**Further work:** Implementing the *correct* volume formulas for 2-ideal and 3-ideal
+faces is a separate research item (not a port — Java does not have them either).
+It is now tracked in `doc/roadmap/research-track.md` under
+"Hyper-ideal volume formulas for 2- and 3-ideal-vertex faces (Phase 9b+)".
+The `throw` remains the correct safe behaviour until that research item is resolved.
+
 ### Acceptance criteria
 - [x] All 8 cases (2³ combinations) are either correct or throw explicitly
 - [x] Three new tests cover the three guard cases