Cover immediate buffer probe failures and document tuning

tools/chatgui-android/app/src/test/java/com/utun/chat/data/CallPlaybackBufferTest.kt

@@ -63,7 +63,7 @@ class CallPlaybackBufferTest {
         assertEquals("stable-probe", d.tick()?.reason)
     }
 
-    @Test fun AcceptedHardwareMinimumIsNotProbedForever() {
+    @Test fun acceptedHardwareMinimumIsNotProbedForever() {
         val d = Device(minimum = 1440)
         d.run(32_000)
         assertEquals(1440, d.frames)
@@ -113,4 +113,28 @@ class CallPlaybackBufferTest {
         d.run(32_000)
         assertEquals(960, d.frames)
     }
+
+    @Test fun firstUnderrunImmediatelyAfterShrinkRejectsProbe() {
+        val d = Device()
+        d.run(32_000)
+        assertEquals(960, d.frames)
+        d.underruns++
+        assertEquals("underrun", d.tick()?.reason)
+        assertEquals(1920, d.frames)
+    }
+
+    @Test fun repeatedFailuresBackOffToFiveMinutesButStillAllowRecovery() {
+        val d = Device()
+        d.run(32_000)
+        val delays = listOf(60_000L, 120_000L, 240_000L, 300_000L, 300_000L)
+        for (delay in delays) {
+            d.underruns++
+            assertEquals("underrun", d.tick()?.reason)
+            assertEquals(1920, d.frames)
+            d.run(delay + 1_800)
+            assertEquals(1920, d.frames)
+            assertEquals("stable-probe", d.tick()?.reason)
+            assertEquals(960, d.frames)
+        }
+    }
 }