fix: a popup save no longer undoes a network switch the background saved during it (closes #641)

The popup's saveState() and the background's updateState() each read the
stored record, change it and write it back, and each was ordered only within
its own context, so a write by one could land between the other's read and
write and be lost. Both now hold the browser's lock (navigator.locks, shared
by every page and the background except a page in a Firefox private window)
from the read to the write, through withLock() in src/shared/browserApi.js;
the recovery screen's reset takes it too. The lock also orders a page's own
saves, so both per-context queues are gone. The unit tests get a small lock
from a Jest setup file.

Model: opus-5-5

README.md

@@ -1389,6 +1389,19 @@ carries a `schemaVersion` — `STATE_SCHEMA_VERSION` in
 `saveState()` and the background's `updateState()` both do, so whichever context
 wrote last, the record says which build's shape it is in.
 
+Each write reads the record, changes it and writes it back, holding a lock from
+the read to the write: the browser's own (`navigator.locks`), taken through
+`withLock()` in `src/shared/browserApi.js`. Every page of the extension and its
+background share it, except a page in a Firefox private window, which Firefox
+gives locks of its own (see https://git.eeqj.de/sneak/AutistMask/issues/646).
+The popup's `saveState()`, the background's `updateState()` and the
+StateRecovery reset all take it, so among the pages and the background that
+share it, no write lands between another write's read and its write, where it
+would be lost. Chrome has it from version 69, older than any Chrome that runs a
+Manifest V3 extension, in pages and in the service worker; Firefox from version
+96, older than the 115 `manifest/firefox.json` declares. Where it is missing, a
+write fails rather than going ahead without it.
+
 Version 1 is the shape that shipped before versions existed, so a stored record
 with no `schemaVersion` is version 1 rather than a defect: it loads normally and
 is migrated in place by being stamped on the first write. An upgrade never shows