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

README.md

@@ -1494,11 +1494,12 @@ 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
+background share it. Firefox gives a page in a private window locks of its own.
+`manifest/firefox.json` sets `"incognito": "not_allowed"`, so such a page cannot
+read or write extension storage. In Chrome, a page opened from an incognito
+window still runs outside incognito and shares the lock. The popup's
+`saveState()`, the background's `updateState()` and the StateRecovery reset all
+take it, so 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