Physics3D: live JoltScene ignores project PhysicsSettings (gravity, timestep, solver/sleep tuning) — only the 3 buffer limits reach it #540

Summary

The runtime physics simulation steps JoltScene (Scene::OnPhysics3DUpdate → m_JoltScene->Simulate(), Scene.cpp:1491), but JoltScene consumes only 3 of the ~20 PhysicsSettings fields. Gravity, fixed timestep, velocity/position solver iterations, Baumgarte, penetration slop, speculative-contact distance, restitution threshold, sleep thresholds, and the determinism / warm-start / manifold-reduction flags are silently ignored by the live simulation. They are applied only to Physics3DSystem::m_PhysicsSystem, a separate JPH::PhysicsSystem that is never stepped for the scene (Physics3DSystem::Update has no caller).

Net effect: a project that authors non-Earth gravity or tunes the solver gets the hardcoded defaults with no warning.

Evidence

• JoltScene::InitializeJolt reads only m_MaxBodies / m_MaxBodyPairs / m_MaxContactConstraints from Physics3DSystem::GetSettings() (JoltScene.cpp:2521-2559), then hardcodes gravity m_JoltSystem->SetGravity(JPH::Vec3(0.0f, -9.81f, 0.0f)) (JoltScene.cpp:2566) and never calls SetPhysicsSettings(...).
• JoltScene::m_FixedTimeStep = 1.0f / 60.0f is a hardcoded member (JoltScene.h:443), not read from settings.m_FixedTimestep.
• JoltScene::SetGravity(...) exists (JoltScene.cpp:262) but has no caller anywhere in the live path.
• Physics3DSystem::UpdatePhysicsSystemSettings() correctly applies gravity + all solver/sleep settings via m_PhysicsSystem->SetGravity(...) / m_PhysicsSystem->SetPhysicsSettings(...) (Physics3DSystem.cpp:486, 513) — but to Physics3DSystem::m_PhysicsSystem, which the scene never simulates (Physics3DSystem::Update, Physics3DSystem.cpp:328, has no caller).

Reproduction

1. Set m_Gravity = {0, -3.71, 0} (Mars) in the project physics settings (persisted by ProjectSerializer).
2. Press Play with any dynamic body.
3. Bodies fall at 9.81 m/s², not 3.71. Likewise raising the velocity-solver iterations, lowering m_PenetrationSlop, or m_AllowSleeping=false has zero effect on the live sim. Only the three buffer limits are honoured (which is exactly what ContactConstraintLimitsHonouredTest checks — it never asserts gravity/solver).

Fix direction

In JoltScene::InitializeJolt, after m_JoltSystem->Init(...), apply the full PhysicsSettings: SetGravity(ToJolt(settings.m_Gravity)), SetPhysicsSettings(...) (mirror Physics3DSystem::UpdatePhysicsSystemSettings), and seed m_FixedTimeStep from settings.m_FixedTimestep. Then either retire the never-stepped Physics3DSystem::m_PhysicsSystem or document its role. Add a test asserting authored gravity/solver settings reach the live JoltScene (the current test only covers the 3 buffer limits).

Score

capability: 5
craft: 3
stability: 5
decay: 1
effort: 2
confidence: 1.0
learning: 2
fun: 3
kano: table-stakes
blocked_by: []
blocks: []

_{Rated per issue-scoring · score = confidence × (capability + craft + stability + decay) / effort, derived by the picker.}