fix: recover when stale preload response resolves after search starts

[bytestream]

When preload: 'focus' is enabled, focusing the dropdown triggers a preload request for the empty query. If the user immediately types a search before the preload responds, a race condition can occur:

1. Focus fires load('') request (preload)
2. User types, fires load('search') request
3. Preload response arrives after the search response
4. The snapshot latch (default_values_loaded) marks itself true despite capturing zero data
5. Clearing the search or closing the dropdown runs restoreDefaults() → addOptions([]) → options wiped permanently
6. preloaded class prevents re-fetch, dropdown stays empty