perf: reduce per-step iteration time (~3.9s to ~2.3s) (#29)

* perf(sidecar): use exec-out + tmpfs for hierarchy dump

Avoids FUSE overhead on /sdcard and shell startup cost by using
exec-out with /data/local/tmp. Saves ~100ms per hierarchy fetch.

* perf(sidecar): replace Thread.sleep waitForIdle with real idle detection

Poll `dumpsys window -a` for mAnimating=true every 50ms instead of
blindly sleeping. Breaks early when device is idle, saving 500-800ms
per step since most settle in <200ms after an action.

* test(sidecar): add idle detection parsing tests

* perf(runner): parallelize hierarchy, metrics, and logs fetch

Run fetchHierarchy, captureMetrics, and collectLogs concurrently via
errgroup so metrics+logs (~150ms) hide behind the hierarchy fetch
(~2s) instead of running serially.

* perf(runner): pipeline post-action screenshot with next step

Defer the post-action screenshot from step N and run it concurrently
with step N+1's hierarchy/metrics/logs fetch. Saves ~335ms per step
by hiding screenshot latency behind the hierarchy fetch.

* test(runner): add tests for parallel fetch and pipelined screenshots

Verify that hierarchy, metrics, and logs are all called per step.
Verify post-action screenshots are written with correct step indices
when pipelined, including the final flush after the loop.

* perf(sidecar): grep mAnimating on-device instead of pulling full dump

The full `dumpsys window -a` output is ~88KB per poll. Running
grep on-device transfers only a count byte, cutting per-poll
overhead from ~63ms to ~56ms and eliminating 88KB of ADB transfer.

sidecar/src/main/kotlin/dev/sanderling/sidecar/DriverBackend.kt

@@ -82,6 +82,11 @@ class StubDriverBackend(private val platform: String) : DriverBackend {
     }
 
     companion object {
+        private const val IDLE_POLL_INTERVAL_MILLIS = 50L
+
+        internal fun isAnimationCountIdle(grepOutput: String): Boolean =
+            (grepOutput.trim().toIntOrNull() ?: 0) == 0
+
         // parseResolvedActivity extracts the activity name from the output of
         // `cmd package resolve-activity --brief`. The brief output is two
         // lines: metadata, then `<pkg>/<activity>`.
@@ -316,8 +321,8 @@ class StubDriverBackend(private val platform: String) : DriverBackend {
         return try {
             val process = ProcessBuilder(
                 listOf(
-                    "adb", "shell",
-                    "uiautomator dump /sdcard/window_dump.xml >/dev/null 2>&1 && cat /sdcard/window_dump.xml",
+                    "adb", "exec-out",
+                    "uiautomator dump /data/local/tmp/window_dump.xml >/dev/null 2>&1 && cat /data/local/tmp/window_dump.xml",
                 ),
             ).redirectErrorStream(false).start()
             val output = process.inputStream.bufferedReader().readText()
@@ -330,9 +335,26 @@ class StubDriverBackend(private val platform: String) : DriverBackend {
     }
 
     override fun waitForIdle(durationMillis: Long) {
-        if (durationMillis > 0) Thread.sleep(durationMillis)
+        if (durationMillis <= 0) return
+        val deadline = System.currentTimeMillis() + durationMillis
+        while (System.currentTimeMillis() < deadline) {
+            if (isDeviceIdle()) return
+            Thread.sleep(IDLE_POLL_INTERVAL_MILLIS)
+        }
     }
 
+    private fun isDeviceIdle(): Boolean {
+        return try {
+            val output = captureAdb(
+                listOf("shell", "dumpsys window -a | grep -c mAnimating=true"),
+            )
+            isAnimationCountIdle(output)
+        } catch (cause: Exception) {
+            false
+        }
+    }
+
+
     override fun healthy(): Boolean = true
 
     // Stateful CPU delta tracker. Reading /proc/<pid>/stat gives cumulative

sidecar/src/test/kotlin/dev/sanderling/sidecar/IdleDetectionTest.kt

@@ -0,0 +1,31 @@
+package dev.sanderling.sidecar
+
+import org.junit.Test
+import kotlin.test.assertFalse
+import kotlin.test.assertTrue
+
+class IdleDetectionTest {
+    @Test fun idleWhenCountIsZero() {
+        assertTrue(StubDriverBackend.isAnimationCountIdle("0\n"))
+    }
+
+    @Test fun idleWhenCountIsZeroNoNewline() {
+        assertTrue(StubDriverBackend.isAnimationCountIdle("0"))
+    }
+
+    @Test fun busyWhenCountIsOne() {
+        assertFalse(StubDriverBackend.isAnimationCountIdle("1\n"))
+    }
+
+    @Test fun busyWhenCountIsMultiple() {
+        assertFalse(StubDriverBackend.isAnimationCountIdle("3\n"))
+    }
+
+    @Test fun idleWhenOutputEmpty() {
+        assertTrue(StubDriverBackend.isAnimationCountIdle(""))
+    }
+
+    @Test fun idleWhenOutputIsNotANumber() {
+        assertTrue(StubDriverBackend.isAnimationCountIdle("error: no service"))
+    }
+}