fix issue where beams were also disappearing while the game was paused

src/ui/GameWorldView.h

@@ -141,11 +141,12 @@ private:
     struct ActiveBeam
     {
         BeamFiredEvent event;
-        qint64    emittedWallMs;
         QVector2D targetOffset;
     };
 
-    static constexpr qint64  kBeamLifetimeMs        = 300;
+    // Beam lifetime in game ticks so beams freeze with the simulation when
+    // paused or slowed, instead of fading on wall-clock time (REQ-SHP-FIRING-BEAM).
+    static constexpr Tick    kBeamLifetimeTicks      = secondsToTicks(0.3);
     static constexpr float   kScrollSpeedTilesPerSec = 10.0f;
 
     Simulation*          m_sim;
@@ -154,7 +155,6 @@ private:
 
     TickDriver    m_tickDriver;
     QElapsedTimer m_frameTimer;
-    qint64        m_wallMs;
     std::mt19937  m_rng;
     double        m_gameSpeedMultiplier;
     double        m_prevNonZeroSpeed;