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

README.md

@@ -1494,15 +1494,16 @@ 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
-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
-write fails rather than going ahead without it.
+background share it. `manifest/firefox.json` sets `"incognito": "not_allowed"`,
+so no page runs in a Firefox private window. Firefox would give a page there
+locks of its own. 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 write
+fails rather than going ahead without it.
 
 Version 1 is the shape that shipped before versions existed, so a stored record
 with no `schemaVersion` is version 1 rather than a defect: it loads normally and