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

errors.go

@@ -5,12 +5,18 @@ import (
 )
 
 var (
-	// When an execution should have data but has none, but the exact error is
-	// indeterminite, this error is returned
-	ErrStdoutEmpty         = errors.New("stdout expected to contain data but was empty")
-	ErrNotInstalled        = errors.New("adb is not installed or not in PATH")
+	// ErrStdoutEmpty is returned when an execution should have data but has none.
+	ErrStdoutEmpty = errors.New("stdout expected to contain data but was empty")
+	// ErrNotInstalled is returned when adb cannot be found in PATH.
+	ErrNotInstalled = errors.New("adb is not installed or not in PATH")
+	// ErrCoordinatesNotFound is returned when touch event coordinates are missing.
 	ErrCoordinatesNotFound = errors.New("coordinates for an input event are missing")
-	ErrConnUSB             = errors.New("cannot call connect/disconnect to device using USB")
+	// ErrConnUSB is returned when connect/disconnect is called on a USB device.
+	ErrConnUSB = errors.New("cannot call connect/disconnect to device using USB")
+	// ErrResolutionParseFail is returned when screen resolution output cannot be parsed.
 	ErrResolutionParseFail = errors.New("failed to parse screen size from input text")
-	ErrUnspecified         = errors.New("an unknown error has occurred, please open an issue on GitHub")
+	// ErrDestExists is returned when a pull destination file already exists.
+	ErrDestExists = errors.New("destination file already exists")
+	// ErrUnspecified is returned when the exact error cannot be determined.
+	ErrUnspecified = errors.New("an unknown error has occurred, please open an issue on GitHub")
 )