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, each 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) from the read to
the write, through withLock() in src/shared/browserApi.js, and the recovery
screen's reset takes it too. A page in a Firefox private window gets locks of
its own and is not covered. 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