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

shell.go

@@ -15,15 +15,14 @@ import (
 // you require functionality not provided by the exposed functions here.
 // Instead of using Shell, please consider submitting a PR with the functionality
 // you require.
-func (d Device) Shell(ctx context.Context, command string) (stdout string, stderr string, ErrCode int, err error) {
+func (d Device) Shell(ctx context.Context, command string) (stdout string, stderr string, errCode int, err error) {
 	cmd, err := shlex.Split(command)
 	if err != nil {
 		return "", "", 1, err
 	}
 	prefix := []string{"-s", string(d.SerialNo), "shell"}
 	cmd = append(prefix, cmd...)
-	stdout, stderr, errcode, err := execute(ctx, cmd)
-	return stdout, stderr, errcode, err
+	return execute(ctx, cmd)
 }
 
 // adb shell wm size