docs(phase5a): student identity and remote classroom documentation

- PHASE5A_STUDENT_IDENTITY_REMOTE_CLASSROOM.md: the full design doc.
- RUNBOOK_STUDENT_ACCOUNT_LIFECYCLE.md, RUNBOOK_REMOTE_CLASSROOM.md: operational runbooks.
- REMOTE_AND_SELF_PACED_ARCHITECTURE.md: closes the 'no real student authentication' gap this
  doc previously flagged as the one item blocking self-paced delivery.
- ENGINEERING_SOP.md §5 rewritten (was describing a pre-Phase-2A state no longer true of any
  deployed version of this codebase) plus a new §14; roadmap item 3 updated.
- LAB_ARCHITECTURE.md, PRODUCTION_VALIDATION_CHECKLIST.md, README.md, platform/README.md:
  cross-references and a Phase 5A production-validation section.

README.md

@@ -150,12 +150,19 @@ its core:
   the browser (`docs/PHASE2A_GATEWAY_AND_STUDENT_FLOW.md`, session model in
   `docs/PHASE3A_IDENTITY_AND_INSTRUCTOR_ACCESS.md` §3, hint gating in
   `docs/PHASE3B_LEARNING_AND_ASSESSMENT.md` §3).
-- The student flow (roster login → curriculum dashboard → Launch Lab → practice/assessments/
-  capstone → achievements) lives in `frontend/src/platform/`; the instructor/admin portal
-  (per-instructor login, cohorts, students, instructor accounts, audit log, classroom
-  "Prepare Labs," environment controls, learning analytics, assessment/capstone management)
-  lives in `frontend/src/admin/` at the `/admin` route — both additions alongside the existing
-  Lab UI, not a rewrite of it.
+- The student flow (real login (email or instructor-assigned login ID) or legacy pilot roster
+  entry → curriculum dashboard → Launch Lab → practice/assessments/capstone → achievements →
+  live classroom → profile) lives in `frontend/src/platform/`; the instructor/admin portal
+  (per-instructor login, cohorts, students with credential/CSV-import management, a remote
+  classroom control center, instructor accounts, audit log, environment controls, learning
+  analytics, assessment/capstone management) lives in `frontend/src/admin/` at the `/admin`
+  route — both additions alongside the existing Lab UI, not a rewrite of it.
+- Students authenticate with real credentials as of Phase 5A (email+password or
+  login-identifier+password, first-login/reset via a single-use token an instructor relays) —
+  see `docs/PHASE5A_STUDENT_IDENTITY_REMOTE_CLASSROOM.md`. Instructors run live, remote-capable
+  classes from `/admin` (presence, Lab/learning status, help-request queue, announcements,
+  guidance-only current-activity) — RootNode owns identity/coordination, an external service
+  (Meet/Zoom/Teams/etc.) owns the actual video/voice.
 
 **Never publish the raw privileged container** — front it with isolation (that's what the
 Gateway is). The one-container `docker run --privileged -p 7554:80 ...` command above stays
@@ -192,7 +199,8 @@ Key properties of the platform (not this image):
 
 Production topology and what's built vs. not yet: `docs/LAB_ARCHITECTURE.md`,
 `docs/PHASE1_PLATFORM_FOUNDATION.md`, `docs/PHASE2A_GATEWAY_AND_STUDENT_FLOW.md`,
-`docs/PHASE3A_IDENTITY_AND_INSTRUCTOR_ACCESS.md`, `docs/PHASE3B_LEARNING_AND_ASSESSMENT.md`.
+`docs/PHASE3A_IDENTITY_AND_INSTRUCTOR_ACCESS.md`, `docs/PHASE3B_LEARNING_AND_ASSESSMENT.md`,
+`docs/PHASE5A_STUDENT_IDENTITY_REMOTE_CLASSROOM.md`.
 
 The production infrastructure package (Proxmox/Ansible/Terraform/firewall/monitoring/backup/CI)
 that runs all of this on real hardware lives in `infra/` — see