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/phase8d_plan.md

@@ -1,6 +1,6 @@
 # Phase 8D — Incremental web administration plan
 
-Status: **8D.0–8D.5 and M1 validated by explicit user sign-off. 8D.5 closed on 2026-09-06 after settled cold-boot evidence and a reported successful 15-minute full-client-mix soak at 230400 baud, with a few broker drops under heavy output. Numeric reserve gates remain open. 8D.6–8D.22 remain planned, each requiring a separate implementation request.** See the [8D.5 implementation record](phase8d5_implementation.md), [8D.4 implementation record](phase8d4_implementation.md), [8D.3 implementation record](phase8d3_implementation.md) and [8D.0 baseline/M1 contract](phase8d_baseline.md).
+Status: **8D.0–8D.5 and M1 validated by explicit user sign-off. 8D.6 implemented, host-tested and build-verified; target validation and independent parent review pending, no phase sign-off. 8D.5 closure on 2026-09-06 stands. Numeric reserve gates remain open. 8D.7–8D.22 remain planned, each requiring a separate implementation request.** See the [8D.6 implementation record](phase8d6_implementation.md), [8D.5 implementation record](phase8d5_implementation.md), [8D.4 implementation record](phase8d4_implementation.md), [8D.3 implementation record](phase8d3_implementation.md) and [8D.0 baseline/M1 contract](phase8d_baseline.md).
 
 This is the execution plan for [roadmap Phase 8D](roadmap.md#phase-8--role-based-users-and-administrative-access). The roadmap retains the feature/security requirements; this document defines small work units, dependencies, and release gates. The [administration test matrix](user_administration_tests.md#planned-phase-8d-integrated-web-administration) remains the final acceptance checklist.
 
@@ -126,6 +126,8 @@ If 8D.3 exceeds the work-unit limit, first split out inert login-page rendering
 
 ### 8D.6 — Browser terminal selector and serial-lease preservation
 
+**Implementation checkpoint (2026-09-06, updated after review fixes):** [8D.6 implementation, accounting and target checklist](phase8d6_implementation.md). Implemented / host-tested / build-verified; target validation and parent re-review of fixes pending, no phase sign-off or M2 completion. Both P2 findings fixed: session identity changes require a clean document before adopting a new view, and fit caches only successful measurements with three bounded readiness retries. Sixteen production-rendered UI groups and focused UI/auth/store reruns pass; prior admin/console regression results remain recorded. Final finite `pio run`: **21.35 s, 95,580 B RAM / 1,646,489 B flash**, **0 / +9,216 B** versus 8D.5; cumulative **+1,048 / +46,516 B** versus 8D.0; review-only delta **0 / +1,376 B**. No backend/capacity/8D.7 restriction/asset change. Numeric reserves remain open; prior sign-offs stand. Stop for validation/user decision before separately requested 8D.7. This supersedes older planned-8D.6 status text in this document.
+
 **Start in:** `src/web_ui.{c,h}`, using the completed serial/admin protocols.
 
 **Scope:** Add admin-only Serial/Admin selection and separate bounded terminal state; no new settings. Keep serial connected and drained/observed while hidden. Keep writer/observer identity and Request control/Release control visible in both modes. Mode changes route displayed output and keyboard input only. Closing the admin route or shell `exit` leaves serial intact; explicit serial Disconnect retains its documented cleanup/reconnect behavior. Bound scrollback and avoid leaking browser listeners/sockets across switches.
@@ -175,11 +177,11 @@ Update the roadmap and user/command documentation to distinguish completed featu
 
 ## Progress and next-request template
 
-Progress: **8D.0–8D.5 and M1 validated by user sign-off; numeric reserves remain open. 8D.6–8D.22 planned.** Record incremental results in `docs/agent/current-state.md`, retaining the [baseline](phase8d_baseline.md) and cumulative resource measurements as work proceeds. The baseline records user-provided evidence and sign-off; this does not imply completion of later browser-authentication acceptance checks.
+Progress: **8D.0–8D.5 and M1 validated by user sign-off; numeric reserves remain open. 8D.6 implemented/host-tested/build-verified, target pending. 8D.7–8D.22 planned.** Record incremental results in `docs/agent/current-state.md`, retaining the [baseline](phase8d_baseline.md) and cumulative resource measurements as work proceeds. The baseline records user-provided evidence and sign-off; this does not imply completion of later browser-authentication acceptance checks.
 
 Suggested next request:
 
-> M1 and 8D.5 are validated by explicit user sign-off. Work on Phase 8D.6 only: implement the browser terminal selector while preserving the serial connection, broker identity and writer lease when hidden. Preserve signed-off evidence and open numeric reserve gates; do not implement 8D.7 lifecycle parity or settings.
+> Review and target-validate Phase 8D.6 using its implementation checklist. Preserve the serial connection, broker identity and writer lease while switching terminals. Record actual browser/target results and open numeric reserve gates; do not implement 8D.7 lifecycle parity or settings without a separate request.
 
 For later chunks: