fix: AutistMask does not work in Firefox private windows (closes #646)

Firefox gives an extension page in a private window locks of its own, so a popup there could save between the background's read and its write and undo a network switch the background saved. `manifest/firefox.json` now sets `"incognito": "not_allowed"`, so such a page cannot read or write extension storage, and `tests/manifest.test.js` pins the key. README and the user guide say AutistMask does not work in Firefox private windows. Chrome needs no change: in its default incognito mode an extension page shares the background's lock.

A Firefox user on "Never remember history" can no longer run AutistMask at all.

Model: opus-5-5

src/background/state.js

@@ -18,11 +18,10 @@
 //   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 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
+//                     the background or by any page (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
@@ -73,8 +72,7 @@ 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, except a page in a Firefox private window, which Firefox
-// gives locks of its own (README, "Stored state and its version").
+// while it is held (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,