feat: adopt lumped mass and reduced-coordinate shell HHT

Integrate Hex8 mass verification and fixed-null-basis shell dynamics with numerical and physics handoff evidence. Preserve interrupted reference and full-validation status; full integration validation is deferred until after performance optimization.

tests/unit/analysis/solid_hht_dynamic_procedure_test.cpp

@@ -304,7 +304,7 @@ TEST(SolidHhtPairedKinematics, InvalidLowInputsFailWithoutChangingPredictor) {
                    .HasValue());
 }
 
-TEST(SolidHhtPairedKinematics, ConsistentMassKeepsSubUlpActionAndCancellation) {
+TEST(SolidHhtPairedKinematics, MassMultiplyKeepsSubUlpActionAndCancellation) {
   fesa::SparsePattern pattern;
   pattern.row_offsets = {0, 2, 4};
   pattern.column_indices = {0, 1, 0, 1};
@@ -569,7 +569,7 @@ TEST(SolidHhtDynamics, RampStartsAtZeroAndAuxiliaryAccelerationIsDistinct) {
 }
 
 TEST(SolidHhtDynamics,
-     OneFreeTranslationHasAnalyticConsistentInertialSupportReactions) {
+     OneFreeTranslationHasAnalyticLumpedInertialSupportReactions) {
   auto result = RunDynamics(OneFree());
   ASSERT_TRUE(result.HasValue()) << solid_hht_test::Message(result.GetStatus());
   const auto d = solid_hht_test::Brick();
@@ -581,12 +581,11 @@ TEST(SolidHhtDynamics,
     const auto& f = frames[n];
     for (unsigned b = 0; b < 8; ++b) {
       std::array<double, 3> product{}, sign{};
-      double scalar_mass = 2.0 / 216;
+      double scalar_mass = b == 0 ? .25 : 0.0;
       for (unsigned c = 0; c < 3; ++c) {
         const bool same = d.nodes[b].coordinates[c] == 0;
         product[c] = same ? 1.0 / 3 : 1.0 / 6;
         sign[c] = same ? -1 : 1;
-        scalar_mass *= same ? 2 : 1;
       }
       for (unsigned c = 0; c < 3; ++c) {
         double stiffness;
@@ -620,7 +619,7 @@ TEST(SolidHhtDynamics,
     }
     if (n != 0) {
       const auto& old = frames[n - 1];
-      const double mass = 2.0 / 27;
+      const double mass = .25;
       const double weighted =
           mass * f.algorithmic_acceleration[0] +
           .95 * (f.internal_force[0] - f.external_force[0]) +
@@ -665,7 +664,7 @@ TEST(SolidHhtDynamics,
     EXPECT_EQ(state.Value().CommittedState().increment_index, 0U);
     EXPECT_DOUBLE_EQ(state.Value().CommittedState().displacement[0], 0);
     EXPECT_NEAR(state.Value().CommittedState().acceleration[0],
-                .01 / (2.0 / 27.0), 1e-14);
+                .01 / .25, 1e-14);
     ASSERT_FALSE(state.Value().IterationRecords().empty());
     EXPECT_EQ(state.Value().IterationRecords().back().stage, "failed");
     EXPECT_FALSE(state.Value().IterationRecords().back().failure_code.empty());