- Serialize first-time id creation with a mutex so concurrent
first fetches cannot mint two UUIDs.
- Fix goimports grouping for the new third-party import.
- Add externalSubSendHwid opt-out (default send); document it.
- Cover header send/omit with httptest in TestFetchSendsStableHwid.
A Master panel fetching a donor subscription sent no X-HWID, so an
HWID-limited donor rejected it with 404. Identify this panel with a
stable per-installation id (persisted in settings), occupying exactly
one donor device slot.
FixesMHSanaei/3x-ui#6559