fix(e2e): run owned-browser background-nav spec last to stop session-poison cascade

owned-browser-background-nav.spec.ts (added in 53aeaa6d7) parents the native
child webview to the shared `home` window via owned_browser_set_bounds. The
whole wdio run shares one app + WebDriver session, and `Window::add_child`
irreversibly drops `home` from the session's window list — `openHomeWindow()`
can never recover the handle. Every spec that sorted after it then failed its
`before` hook with "Could not get home window handle", cascading 10 failures on
macOS AND Windows (Linux escaped only because it skips the spec). The commit's
"Windows uses WebView2 and is also fine" assumption was wrong — green Linux hid
that both desktop platforms were red.

Rename it `zz-…` so alphabetical spec order puts it last (after `windows-*`),
where poisoning `home` harms nothing. The product behavior under test is
unchanged and still asserted on macOS + Windows. Header documents why it must
stay last; Rust/bindings doc-comments updated to the new filename.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>