implement simulation shell

src/lib/sim/TickDriver.h

@@ -0,0 +1,21 @@
+#pragma once
+
+// Accumulator-based fixed-timestep driver. Decouples wall-clock render rate
+// from the 30 Hz simulation tick rate. See architecture.md §Render Loop.
+class TickDriver
+{
+public:
+    TickDriver() = default;
+
+    // Adds elapsedWallMs * gameSpeedMultiplier to the accumulator and returns
+    // how many simulation ticks should be stepped this frame. The remainder
+    // stays in the accumulator for the next call.
+    // A multiplier of 0.0 freezes accumulation (pause).
+    // Valid multipliers per REQ-UI-SPEED: 0, 0.5, 1, 2, 4.
+    int advance(double elapsedWallMs, double gameSpeedMultiplier);
+
+    void reset();
+
+private:
+    double m_accumulatorMs = 0.0;
+};