physics: add server swim-back speed override to CameraController

Backward swimming was using 50% of forward swim speed as a hardcoded
fallback. Wire up the server-authoritative swim back speed so Warlock
Dark Pact, buffs, and server-forced speed changes all apply correctly
when swimming backward.

- game_handler.hpp: add getServerSwimBackSpeed() accessor
- camera_controller.hpp: add swimBackSpeedOverride_ field + setter
- camera_controller.cpp: apply swimBackSpeedOverride_ when player
  swims backward without forward input; fall back to 50% of swim speed
- application.cpp: sync swim back speed each frame

include/game/game_handler.hpp

@@ -1154,6 +1154,7 @@ public:
     float getServerRunSpeed() const { return serverRunSpeed_; }
     float getServerWalkSpeed() const { return serverWalkSpeed_; }
     float getServerSwimSpeed() const { return serverSwimSpeed_; }
+    float getServerSwimBackSpeed() const { return serverSwimBackSpeed_; }
     float getServerFlightSpeed() const { return serverFlightSpeed_; }
     float getServerRunBackSpeed() const { return serverRunBackSpeed_; }
     bool isPlayerRooted() const {

include/rendering/camera_controller.hpp

@@ -94,6 +94,7 @@ public:
     void setRunSpeedOverride(float speed) { runSpeedOverride_ = speed; }
     void setWalkSpeedOverride(float speed) { walkSpeedOverride_ = speed; }
     void setSwimSpeedOverride(float speed) { swimSpeedOverride_ = speed; }
+    void setSwimBackSpeedOverride(float speed) { swimBackSpeedOverride_ = speed; }
     void setFlightSpeedOverride(float speed) { flightSpeedOverride_ = speed; }
     void setRunBackSpeedOverride(float speed) { runBackSpeedOverride_ = speed; }
     void setMovementRooted(bool rooted) { movementRooted_ = rooted; }
@@ -281,6 +282,7 @@ private:
     float runSpeedOverride_ = 0.0f;
     float walkSpeedOverride_ = 0.0f;
     float swimSpeedOverride_ = 0.0f;
+    float swimBackSpeedOverride_ = 0.0f;
     float flightSpeedOverride_ = 0.0f;
     float runBackSpeedOverride_ = 0.0f;
     // Server-driven root state: when true, block all horizontal movement input.

src/core/application.cpp

@@ -1011,6 +1011,7 @@ void Application::update(float deltaTime) {
                     renderer->getCameraController()->setRunSpeedOverride(gameHandler->getServerRunSpeed());
                     renderer->getCameraController()->setWalkSpeedOverride(gameHandler->getServerWalkSpeed());
                     renderer->getCameraController()->setSwimSpeedOverride(gameHandler->getServerSwimSpeed());
+                    renderer->getCameraController()->setSwimBackSpeedOverride(gameHandler->getServerSwimBackSpeed());
                     renderer->getCameraController()->setFlightSpeedOverride(gameHandler->getServerFlightSpeed());
                     renderer->getCameraController()->setRunBackSpeedOverride(gameHandler->getServerRunBackSpeed());
                     renderer->getCameraController()->setMovementRooted(gameHandler->isPlayerRooted());

src/rendering/camera_controller.cpp

@@ -537,6 +537,10 @@ void CameraController::update(float deltaTime) {
             // Use character's facing direction for strafe, not camera's right vector
             glm::vec3 swimRight = right;  // Character's right (horizontal facing), not camera's
 
+            float swimBackSpeed = (swimBackSpeedOverride_ > 0.0f && swimBackSpeedOverride_ < 100.0f
+                                    && !std::isnan(swimBackSpeedOverride_))
+                                       ? swimBackSpeedOverride_ : swimSpeed * 0.5f;
+
             glm::vec3 swimMove(0.0f);
             if (nowForward) swimMove += swimForward;
             if (nowBackward) swimMove -= swimForward;
@@ -545,7 +549,9 @@ void CameraController::update(float deltaTime) {
 
             if (glm::length(swimMove) > 0.001f) {
                 swimMove = glm::normalize(swimMove);
-                targetPos += swimMove * swimSpeed * physicsDeltaTime;
+                // Use backward swim speed when moving backwards only (not when combining with strafe)
+                float applySpeed = (nowBackward && !nowForward) ? swimBackSpeed : swimSpeed;
+                targetPos += swimMove * applySpeed * physicsDeltaTime;
             }
 
             // Spacebar = swim up (continuous, not a jump)