refactor(diagnostics): split the flap window and episode rule apart

detekt ReturnCount. Extracting the pruning and the is-this-an-episode
predicate reads better than suppressing it, and the cooldown rule now
has a name and a docstring of its own.

android/app/src/main/java/com/fabledsword/minstrel/diagnostics/TransportFlapDetector.kt

@@ -33,20 +33,30 @@ class TransportFlapDetector(
      */
     fun onChange(observation: TransportObservation): List<TransportObservation>? {
         recent.addLast(observation)
-        while (recent.isNotEmpty() &&
-            observation.atElapsedMs - recent.first().atElapsedMs > windowMs
-        ) {
-            recent.removeFirst()
-        }
-        if (recent.size < minChanges) return null
-        // One episode, one summary. A sustained fault would otherwise emit a
-        // summary per reading and bury the per-change events underneath them.
-        val since = lastSummaryAtMs
-        if (since != null && observation.atElapsedMs - since < summaryCooldownMs) return null
+        dropReadingsOlderThan(observation.atElapsedMs)
+        if (!isEpisode(observation.atElapsedMs)) return null
         lastSummaryAtMs = observation.atElapsedMs
         return recent.toList()
     }
 
+    private fun dropReadingsOlderThan(nowMs: Long) {
+        while (recent.isNotEmpty() && nowMs - recent.first().atElapsedMs > windowMs) {
+            recent.removeFirst()
+        }
+    }
+
+    /**
+     * Enough changes packed together, and far enough from the last thing we
+     * wrote down. The cooldown is what keeps one episode to one summary: a
+     * sustained fault produces a change every poll, and a summary per reading
+     * would bury the per-change events underneath them.
+     */
+    private fun isEpisode(nowMs: Long): Boolean {
+        val since = lastSummaryAtMs
+        val cooled = since == null || nowMs - since >= summaryCooldownMs
+        return recent.size >= minChanges && cooled
+    }
+
     /** Forget everything — call when the route changes or casting ends. */
     fun reset() {
         recent.clear()