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/architecture.md

@@ -116,6 +116,10 @@ Web serial initialization is failure-isolated from the base HTTPS service: if th
 
 ### Browser admin backend
 
+The 8D.6 document binds retained terminal state to its first validated username/role/session-stable CSRF tuple. Every later session adoption must match, otherwise both terminal hosts are hidden, both sockets/work are closed/fenced and a clean `/` document is required. Pagehide hides scrollback until same-session revalidation; no clearing is needed for unchanged-session restore or mode changes. Terminal-fit readiness uses successful-bounds caching and at most three generation-fenced animation-frame retries per external request, never unbounded polling.
+
+8D.6 `web_ui.c` now supplies the admin-only Serial/Admin selector using this backend. Selection leaves serial and any open admin socket connected and draining; only focus, displayed terminal and keyboard destination change. Broker identity/lease and writer controls remain serial-owned in both views. Admin opens/reopens explicitly and closes independently. Two page-lifetime terminals have separate 5,000-line scrollbacks and 64 KiB callback-accounted pending output each; browser overflow is dropped with visible counts. Admin input is bounded to 4 KiB admission and 512-byte frames. Logout/expiry/page exit closes both with generation fencing and socket-listener cleanup; bfcache revalidates serial/session but never automatically reopens admin. No server policy/capacity changes or 8D.7 lifecycle parity. See `docs/phase8d6_implementation.md`; the following paragraph describes the original backend boundary, before its UI entry was added.
+
 8D.5 additionally supplies `web_admin_transport` and `web_admin_tickets`: one optional admin socket, two 30-second digest-only tickets bound to current originating session/principal, the same two shared console slots, no serial broker client. Ticket POST requires cookie/Origin/CSRF/admin; ordinary GET upgrade requires cookie/Origin/admin/ticket and console admission before 101. Six total HTTPS sockets remain, LRU purge is disabled, and two routes bring the handler budget to 16. Optional admin registration/PSRAM allocation failures do not take down M1. A 20 ms ESP timer queues at most one HTTPD poll, with no new task; only HTTPD accesses the 1,552-byte PSRAM-only RX/TX payload or socket IO. Notifiers close the generation-qualified console and flag the socket. HTTPD shuts down the verified current fd directly and owns subsequent read cleanup, avoiding IDF's queued reusable `sock_db *` close race. Detach fences submissions; failed stop retains ownership, and queued state is retired only after successful HTTPD stop. Console dispatcher/prompt and owner input/output/idle checks enforce session and principal currentness. WEB supports deferred self-close only; parsed canonical policy denies unsupported lifecycle/network/account mutations before handler side effects. No normal UI entry, typed settings or lifecycle parity is included. See `docs/phase8d5_implementation.md` for validation limits and exact restrictions.
 
 ### SSH