add basic types and fix cmake

src/lib/core/Tick.h

@@ -0,0 +1,23 @@
+#pragma once
+
+// Discrete simulation time, measured in ticks since t=0.
+using Tick = long long;
+
+// Fixed simulation tick rate. See architecture.md "Fixed-Timestep Tick-Based Simulation".
+constexpr int kTickRateHz = 30;
+constexpr double kTickDurationMs = 1000.0 / kTickRateHz;
+constexpr double kTickDurationSeconds = 1.0 / kTickRateHz;
+
+// Converts a wall-clock duration (in seconds, as it appears in config TOML) to
+// an integer tick count. Rounds to nearest to avoid systematic drift from
+// repeated conversions.
+constexpr Tick secondsToTicks(double seconds)
+{
+    return static_cast<Tick>(seconds * kTickRateHz + 0.5);
+}
+
+// Inverse of secondsToTicks; useful for logging and UI display.
+constexpr double ticksToSeconds(Tick ticks)
+{
+    return static_cast<double>(ticks) / kTickRateHz;
+}