test(US-03): add failing tests for 880 Hz beep generation

app/src/test/kotlin/net/jeena/pacer/BeepGeneratorTest.kt

@@ -0,0 +1,50 @@
+package net.jeena.pacer
+
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertTrue
+import org.junit.Test
+import kotlin.math.abs
+
+class BeepGeneratorTest {
+
+    @Test
+    fun `generateBeep returns correct number of samples`() {
+        val durationMs = 50
+        val samples = BeepGenerator.generateBeep(durationMs)
+        val expected = BeepGenerator.SAMPLE_RATE * durationMs / 1000
+        assertEquals(expected, samples.size)
+    }
+
+    @Test
+    fun `generateBeep samples are within Short range`() {
+        val samples = BeepGenerator.generateBeep(50)
+        assertTrue(samples.all { it in Short.MIN_VALUE..Short.MAX_VALUE })
+    }
+
+    @Test
+    fun `generateBeep produces non-silent output`() {
+        val samples = BeepGenerator.generateBeep(50)
+        val maxAmplitude = samples.maxOf { abs(it.toInt()) }
+        assertTrue("Expected non-silent beep, max amplitude was $maxAmplitude", maxAmplitude > 1000)
+    }
+
+    @Test
+    fun `generateBeep produces approximately 880 Hz frequency`() {
+        val durationMs = 100
+        val samples = BeepGenerator.generateBeep(durationMs)
+
+        // Count zero crossings (positive→negative transitions)
+        var crossings = 0
+        for (i in 1 until samples.size) {
+            if (samples[i - 1] > 0 && samples[i] <= 0) crossings++
+        }
+
+        // At 880 Hz over 100 ms we expect ~88 complete cycles = ~88 crossings
+        val expectedCrossings = BeepGenerator.FREQUENCY * durationMs / 1000
+        val tolerance = expectedCrossings * 0.05 // 5% tolerance
+        assertTrue(
+            "Expected ~$expectedCrossings zero crossings, got $crossings",
+            abs(crossings - expectedCrossings) < tolerance
+        )
+    }
+}