Add browser Serial/Admin terminal switching

Keep the serial connection and lease intact while providing a separate,
bounded admin terminal with explicit open and close controls. Fence
retained
terminal state across sessions and add fit-readiness retries with
regression
coverage.

docs/agent/design-decisions.md

@@ -100,6 +100,8 @@ Phase 8D.4 routes drain/lifecycle operations through a firmware-lifetime immutab
 
 ## Browser authentication has a narrow version-pinned HTTPD boundary
 
+**8D.6 terminal separation:** Browser selection never reconnects serial or requests/releases a writer lease. Hidden connected terminals continue draining with separate bounded scrollback/pending writes and visible browser-drop accounting; only selected keyboard input is sent. Admin admission/reopen is explicit, close is isolated, and logout/expiry/pagehide tears down both routes. Keep the two page-lifetime input subscriptions stable across switches and remove socket callbacks on close. UI role hiding complements, never replaces, backend authorization. Existing unsupported WEB lifecycle/account-command restrictions remain for 8D.7.
+
 **Decision:** 8D.3 uses `web_cookie_auth` plus digest-only session/challenge stores, mandatory Origin/CSRF mutations and no live session/challenge/ticket eviction. Four one-hour absolute sessions deliberately interrupt long serial connections at expiry. No Basic compatibility path remains.
 
 **Browser Origin serialization:** Authentication POST fetches use `mode: 'cors'` while retaining fixed same-origin URLs, `credentials: 'same-origin'`, redirect rejection and CSP `connect-src 'self'`. Under `no-referrer`, non-CORS POST mode can serialize Origin as `null` (confirmed in Firefox during M1 testing). Do not fix that by accepting null server-side or weakening CSP/referrer policy; no cross-origin server permission is added.