test(US-04): add failing tests for BPM interval calculation and volume clamping

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

@@ -0,0 +1,42 @@
+package net.jeena.pacer
+
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertTrue
+import org.junit.Test
+
+class BpmCalculatorTest {
+
+    @Test
+    fun `intervalMs returns 1000ms for 60 BPM`() {
+        assertEquals(1000L, BpmCalculator.intervalMs(60))
+    }
+
+    @Test
+    fun `intervalMs returns 500ms for 120 BPM`() {
+        assertEquals(500L, BpmCalculator.intervalMs(120))
+    }
+
+    @Test
+    fun `intervalMs returns correct value for 130 BPM`() {
+        // 60000 / 130 = 461ms
+        assertEquals(461L, BpmCalculator.intervalMs(130))
+    }
+
+    @Test
+    fun `intervalMs returns correct value for 80 BPM`() {
+        assertEquals(750L, BpmCalculator.intervalMs(80))
+    }
+
+    @Test
+    fun `clampVolume keeps value within 0 to 1`() {
+        assertEquals(0f, BpmCalculator.clampVolume(-0.5f))
+        assertEquals(1f, BpmCalculator.clampVolume(1.5f))
+        assertEquals(0.7f, BpmCalculator.clampVolume(0.7f))
+    }
+
+    @Test
+    fun `intervalMs decreases as BPM increases`() {
+        assertTrue(BpmCalculator.intervalMs(60) > BpmCalculator.intervalMs(120))
+        assertTrue(BpmCalculator.intervalMs(120) > BpmCalculator.intervalMs(180))
+    }
+}