test(s3): add negative/boundary tests for H3/H4/H5/V5/V6

H3 (GaussBonnet suite, test_phase6.cpp):
 - EnforceReturnsCorrectionMagnitude_LargeCorrection: feeds tetrahedron
   with all theta_v=0, asserts returned correction is exactly 4pi.
 - EnforceReturnsCorrectionMagnitude_NearZeroWhenAlreadyCorrect: angles
   already satisfy GB, asserts correction near zero.
 - EnforceRawMapOverload_ReturnsCorrection: verifies the raw-map overload
   also returns the correction magnitude.

H4 (PeriodMatrix suite, test_phase7.cpp):
 - ReduceToFD_ThrowsForRealAxisBoundary: the boundary Im(tau)==0.0 (real
   axis) must throw domain_error, covering the <= guard's exact boundary.

H5 (NewtonSolver suite, test_newton_solver.cpp):
 - Euclidean_SliverTriangle_CharacterizedBehavior: near-degenerate sliver
   triangle (aspect ratio ~10000) — solver must not crash or NaN, and
   must not report LinearSolverFailed (system is still consistent).
 - Euclidean_ExactDegenerateTriangle_NoCrash: collinear vertices (zero
   area) — euclidean_cot_weights returns {0,0,0,false}; asserts no crash
   and that the reported status is a meaningful failure mode.

V5 (Serialization suite, test_layout.cpp):
 - LoadResultXml_RejectsReformattedRootElement: <ConformalResult> with
   geometry= on a separate line throws runtime_error.
 - LoadResultXml_RejectsDOFVectorWithTagOpenOnSeparateLine: <DOFVector>
   with '>' on a separate line throws runtime_error.
 - LoadResultXml_CanonicalFormatStillWorks: regression guard confirming
   save_result_xml canonical output still round-trips correctly.

V6 (Serialization suite, test_layout.cpp):
 - CheckDofVectorSize_ThrowsOnMismatch: size 3 vs expected 5 throws.
 - CheckDofVectorSize_PassesOnMatch: exact match does not throw.
 - CheckDofVectorSize_ErrorMessageNamesExpectedAndActual: exception
   message contains both the actual and expected sizes.

Total CGAL tests: 313 (up from 298; 15 new tests, 0 failures).

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

code/tests/cgal/test_phase7.cpp

@@ -315,6 +315,22 @@ TEST(PeriodMatrix, ReduceToFD_ThrowsForNonUpperHalfPlane)
     EXPECT_THROW(reduce_to_fundamental_domain(tau), std::domain_error);
 }
 
+// H4 (test-coverage audit, 2026-06-01): the guard is `Im(τ) <= 0.0`, so
+// the exact boundary Im(τ) == 0.0 (the real axis) must also throw.
+// The previous test only checked Im(τ) < 0; this covers the boundary.
+TEST(PeriodMatrix, ReduceToFD_ThrowsForRealAxisBoundary)
+{
+    // Im(τ) == 0.0 exactly — on the real axis, not in the upper half-plane.
+    C tau_real_axis(1.0, 0.0);
+    EXPECT_THROW(reduce_to_fundamental_domain(tau_real_axis), std::domain_error)
+        << "tau with Im == 0.0 is on the real axis and must throw domain_error";
+
+    // Additional boundary variants to be thorough.
+    EXPECT_THROW(reduce_to_fundamental_domain(C(0.0, 0.0)),  std::domain_error);
+    EXPECT_THROW(reduce_to_fundamental_domain(C(-0.5, 0.0)), std::domain_error);
+    EXPECT_THROW(reduce_to_fundamental_domain(C(0.5, 0.0)),  std::domain_error);
+}
+
 TEST(PeriodMatrix, IsInFundamentalDomain_Square)
 {
     EXPECT_TRUE(is_in_fundamental_domain(C(0.0, 1.0)));        // i