docs(phase5b): self-paced enrollment documentation + website integration contract

- PHASE5B_SELF_PACED_ENROLLMENT.md: the full design doc.
- RUNBOOK_SELF_PACED_ENROLLMENT.md: operational runbook for /admin -> Programs.
- WEBSITE_ENROLLMENT_INTEGRATION.md: the complete contract for the separate website team —
  endpoint, auth, request/response schema, idempotency/retry rules, student matching, minor/
  no-email behavior, error responses, what not to send, local testing, production-secret
  handling. Written to be usable without reading the Platform API source.
- REMOTE_AND_SELF_PACED_ARCHITECTURE.md: closes the self-registration/curriculum-assignment/
  delivery-mode/enrollment-model gaps this doc previously flagged as Phase 5+ scope.
- ENGINEERING_SOP.md: new §15. LAB_ARCHITECTURE.md, PRODUCTION_VALIDATION_CHECKLIST.md,
  README.md, platform/README.md: cross-references and a Phase 5B production-validation section.

docs/LAB_ARCHITECTURE.md

@@ -507,3 +507,15 @@ any of it — the Provisioner, the `environments` state machine, the Lab Host Ro
 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.
+
+## Phase 5B: still reused, not modified — a gate in front, not a second Lab
+
+Phase 5B (`docs/PHASE5B_SELF_PACED_ENROLLMENT.md`) added self-paced programs/enrollment on top
+of the exact same disposable Lab this document describes — again without changing the
+Provisioner, `environments`, the reaper, or the Lab Host Router at all. A self-paced launch
+(`POST /api/student/programs/:id/launch`) is an entitlement check followed by calling the
+identical launch sequence `/api/environment/launch` always has — verified for real by killing a
+Lab container out-of-band and restarting the Provisioner to trigger its actual
+`reconcileOnStartup` reconciliation path (the same technique Phase 4.1's
+`test-lab-host-loss.js` established), then confirming a self-paced student's academic progress
+is completely unaffected and a fresh Lab boots normally.