fix: AutistMask does not run 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 Firefox never
runs AutistMask in a private window. tests/manifest.test.js pins the key.
The README and five code comments no longer describe a private-window
exception to the lock.

Model: opus-5-5

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");