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.

go.mod

@@ -6,6 +6,7 @@ require (
 	github.com/dop251/goja v0.0.0-20260311135729-065cd970411c
 	github.com/evanw/esbuild v0.28.0
 	github.com/fsnotify/fsnotify v1.9.0
+	golang.org/x/sync v0.20.0
 	google.golang.org/grpc v1.80.0
 	google.golang.org/protobuf v1.36.11
 )