Implement playable Mac client and rendering validation

extension/tests/water_motion_test.cpp

@@ -0,0 +1,21 @@
+#include "water_motion.h"
+#include <cassert>
+#include <cmath>
+#include <cstdio>
+int main() {
+	mtgodot::WaterMotion m;
+	assert(m.sample(100, 2000, 15) == 0);
+	assert(m.sample(400, 2000, 15) == 0);
+	assert(m.sample(401, 2000, 15) == 0);
+	assert(std::abs(m.sample(1401, 2000, 15) + .075) < 1e-9);
+	assert(std::abs(m.sample(2401, 2000, 15) + .15) < 1e-9);
+	assert(std::abs(m.sample(2402, 1000, 15) + .15) < 1e-9);
+	assert(std::abs(m.sample(2902, 1000, 15) + .075) < 1e-9);
+	assert(m.sample(3402, 1000, 15) == 0);
+	// Large frame gaps start at the last rendered height, without overshoot.
+	for (uint64_t t = 3403; t < 200000; t += 137) {
+		const double y = m.sample(t, 1700, 9);
+		assert(y <= 0 && y >= -.15);
+	}
+	std::puts("PASS: shared linear water motion, endpoints, timing, no overshoot");
+}