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

src/background/state.js

@@ -17,12 +17,13 @@
 //                     move it.
 //   updateState(fn)   read fresh, apply fn to that fresh record, write it
 //                     back — all holding the lock every write of the stored
-//                     record takes, so no other write, by the background or a
-//                     page, lands in between, and the read is one storage
-//                     round trip ahead of the write rather than a page
-//                     lifetime ahead of it (which is what made the popup's
-//                     saveState() need a per-field merge against a baseline
-//                     at all).
+//                     record takes, so no other write lands in between, by
+//                     the background or by any page except a page in a
+//                     Firefox private window (README, "Stored state and its
+//                     version"). The read is one storage round trip ahead of
+//                     the write rather than a page lifetime ahead of it
+//                     (which is what made the popup's saveState() need a
+//                     per-field merge against a baseline at all).
 //
 // A handler that must both read and write therefore does its network work
 // against a snapshot it owns, and applies the RESULT inside updateState().
@@ -72,7 +73,8 @@ async function updateStateOnce(mutate) {
 // src/shared/browserApi.js) is held from the read to the write: two
 // background turns, or a turn and a page's save, never interleave
 // (https://git.eeqj.de/sneak/AutistMask/issues/641). Every page's save waits
-// while it is held.
+// while it is held, except a page in a Firefox private window, which Firefox
+// gives locks of its own (README, "Stored state and its version").
 //
 // `mutate` must also not call updateState() itself, directly or through
 // anything it awaits: the inner turn waits for the lock the outer one holds,

src/shared/state.js

@@ -572,7 +572,9 @@ function reportSaveFailure(err) {
 // Each save runs start to finish holding the lock every write of the stored
 // record takes (withLock() in src/shared/browserApi.js), so no write by
 // another page or the background lands between its read and its write and is
-// lost (https://git.eeqj.de/sneak/AutistMask/issues/641).
+// lost (https://git.eeqj.de/sneak/AutistMask/issues/641). A page in a Firefox
+// private window is the exception: Firefox gives it locks of its own (README,
+// "Stored state and its version").
 //
 // The same lock orders this page's own saves. showView() calls saveState() on
 // every navigation without awaiting it, so two saves from one page can be