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.

docs/LAB_ARCHITECTURE.md

@@ -498,3 +498,12 @@ mechanism for reaching a Lab container (it assumes Gateway and Lab share a Docke
 breaks once they're genuinely on separate VMs); Phase 4.1
 (`docs/PHASE4_1_PREPRODUCTION_HARDENING.md`) built and simulation-tested the fix, a Lab Host
 Router (`platform/lab-router/`).
+
+## Phase 5A: reused, not modified
+
+Phase 5A (`docs/PHASE5A_STUDENT_IDENTITY_REMOTE_CLASSROOM.md`) added real student accounts and a
+remote classroom control center on top of everything this document describes, without changing
+any of it — the Provisioner, the `environments` state machine, the Lab Host Router, disposable
+Lab lifecycle, and the Platform/Lab-Host VM boundary are all untouched. The classroom dashboard
+reads Lab status the exact same way every other admin view does (`provisioner.listForStudent`,
+the same state enum) rather than inventing a second one.