observe

Returns a cold Flow that emits the current value of key and then re-emits whenever that key is written, removed, or the vault is cleared.

Semantics:

• The first emission is the value present at collection start (or null).

• Subsequent emissions are de-duplicated against the previously emitted value — collectors only see real changes.

• The flow never completes on its own; cancel the collecting coroutine to unregister the underlying listener.

Platform notes:

• Android — backed by SharedPreferences.OnSharedPreferenceChangeListener; sees writes from any code holding the same prefs file.

• iOS — backed by an in-process broker. Writes performed from another process, or via raw SecItem* calls outside this library, are not observed.

Throws VaultException.InvalidKey at collection time if key is blank.