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

Firefox gives an extension page in a private window locks of its own. A
popup there could save between the background's read and its write of the
stored record, and undo a network switch the background had saved.

manifest/firefox.json now sets "incognito": "not_allowed", so a page in a
private window cannot read or write extension storage. tests/manifest.test.js
pins the key. The README says this in "Stored state and its version". Six
code comments drop their private-window exception to the lock.
docs/README.md tells Firefox users that AutistMask does not work in private
windows.

Model: opus-5-5

tests/manifest.test.js

@@ -165,6 +165,15 @@ describe("declared icons", () => {
     });
 });
 
+// Firefox gives a page in a private window locks of its own, so that page's
+// save could land between the background's read and its write (README,
+// "Stored state and its version").
+describe("private windows", () => {
+    test("a page in a firefox private window cannot read or write extension storage", () => {
+        expect(readManifest("firefox").incognito).toBe("not_allowed");
+    });
+});
+
 describe("shipped Content Security Policy", () => {
     // MV3 takes an object and applies extension_pages to the popup and the
     // background service worker, which is where libsodium runs.

tests/stateWriteLock.test.js

@@ -6,9 +6,8 @@
 // now holds a lock from its read to its write.
 //
 // The popup and the background run here as the real modules over one shared
-// storage stub, and share the lock the way the background and a page outside
-// a Firefox private window share the browser's
-// (tests/support/navigatorLocks.js).
+// storage stub, and share the lock the way the background and its pages share
+// the browser's (tests/support/navigatorLocks.js).
 
 const { makeStorageStub } = require("./support/storageStub");
 

tests/support/navigatorLocks.js

@@ -4,9 +4,8 @@
 // the callback settles, whether it resolved or rejected. That is what the
 // browser's own does for the one call the extension makes,
 // `navigator.locks.request(name, callback)`. Every module a test file loads
-// shares it, as the extension's background and its pages share the browser's,
-// except a page in a Firefox private window (README, "Stored state and its
-// version").
+// shares it, as the extension's background and its pages share the browser's
+// (README, "Stored state and its version").
 //
 // Run before each test file by `setupFiles` in package.json.