fix: replace panic with error return in execute(), fix Pull/Push/Reboot/Root targeting

- execute() no longer panics when adb is not found; returns ErrNotInstalled
- Use sync.Once for lazy adb path lookup instead of init()
- Fix Pull() checking src (device path) instead of dest (local path)
- Add ErrDestExists error for Pull destination conflicts
- Add -s serial flag to Push, Pull, Reboot, Root for multi-device support
- Root() now parses stdout to determine actual success
- Fix Shell() redundant variable redeclaration
- Fix 'inconsostency' typo in KillServer doc
- Add doc comments to all error variables
- Update Go 1.26.0 -> 1.26.1
- Add tests: ConnString, ShortenSleep, GetLength, JSON roundtrip,
  SequenceImporter.ToInput, insertSleeps, parseDevices edge cases,
  parseScreenResolution edge cases, filterErr

util_test.go

@@ -0,0 +1,24 @@
+package adb
+
+import (
+	"testing"
+)
+
+func Test_filterErr(t *testing.T) {
+	tests := []struct {
+		name    string
+		stderr  string
+		wantErr bool
+	}{
+		{name: "empty stderr", stderr: "", wantErr: false},
+		{name: "random output", stderr: "some warning text", wantErr: false},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := filterErr(tt.stderr)
+			if (err != nil) != tt.wantErr {
+				t.Errorf("filterErr() error = %v, wantErr %v", err, tt.wantErr)
+			}
+		})
+	}
+}