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/code-map.md

@@ -73,6 +73,7 @@ This is a semantic map, not a complete file inventory. Start here, then read the
 - 8D.3 HTTP policy: `web_cookie_auth` owns public login/challenge/login POST/session/logout routes and protected-route checks; `web_auth_parse` handles bounded values/JSON. `web_httpd_adapter` alone reads private IDF 5.5.0 header scratch, rejects duplicate fields, defers 101 until transport admission and wipes consumed scratch while preserving right-aligned pending bytes. No SDK patch. `src/CMakeLists.txt` supplies private includes and compiles HTTPD warning/debug logs out. Test with `python3 tests/web_cookie_auth/run.py` and `python3 tests/web_auth_parse/run.py`.
 - 8D.3 UI: `src/web_login_ui.{c,h}` serves standalone `/login`; `web_ui.c` validates session before serial connect/restore and handles logout/401 safely. Both scripts hash-bound, auth documents/app no-store. Tests: `python3 tests/web_login_ui/run.py` and `python3 tests/web_ui_session/run.py`. Live cutover host-tested/build-verified, M1 validated by user sign-off (numeric reserves open): `docs/phase8d3_implementation.md`.
 - Asset constraint: `web_assets_data.c` is checked-in generated input to the build; do not hand-edit or regenerate casually.
+- 8D.6 UI: `web_ui.c` adds admin-only Serial/Admin selection and explicit admin open/close through existing endpoints. Serial socket/client/lease survives mode switches; hidden output drains into independent 5,000-line/64 KiB-pending terminals with visible browser-drop counts. Selected keyboard only; logout/expiry/pagehide closes both with handler cleanup. Session identity changes require a clean document before adopting the view; same-session restore retains hidden-until-validated buffers. Fit readiness retries are bounded to three and cache only success. Focused `tests/web_ui_session/run.py` now has 16 groups; target checks remain pending in `docs/phase8d6_implementation.md`. 8D.5 is user-validated; no 8D.7 restriction change.
 
 ### Browser admin backend (8D.5)