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/RUNBOOK_SELF_PACED_ENROLLMENT.md

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+# Runbook: Self-Paced Programs & Enrollment
+
+Covers managing programs and enrollments from `/admin` → Programs. See
+`docs/PHASE5B_SELF_PACED_ENROLLMENT.md` for the underlying design and
+`docs/WEBSITE_ENROLLMENT_INTEGRATION.md` for the separate website-driven path.
+
+**Create a program** (Programs → Create program, admin/owner only):
+
+Program ID is a slug (e.g. `root-linux-fundamentals`) and can't be changed later. Pick a
+family (`init_`/`compile_`/`root_`) and delivery mode. New programs default to
+`curriculum_path: rootnode-path` — the only curriculum content that exists today. A program for
+`init_`/`compile_` can be created now as a placeholder and wired to real content later without a
+schema change; it just won't have real lessons behind it yet.
+
+**Activate/deactivate a program** (Programs → \[program\] → Activate/Deactivate):
+
+An inactive program can't accept new enrollments (admin-manual or via the website integration
+endpoint) but existing enrollments in it are untouched — students already enrolled keep their
+access exactly as it was.
+
+**Manually enroll a student** (Programs → \[program\] → Enroll a student):
+
+Enter the student's ID (find it on the Students tab). Choose `pending` (the default — the
+student sees the enrollment but can't start until you Activate it) or `active` (skip that step).
+Optional start/end dates enforce access windows server-side — a future start date hides Lab
+access until then; an end date cuts off new Lab launches after it while leaving prior progress
+visible.
+
+**Activate a pending enrollment** (Programs → \[program\] → Enrollments table → Activate):
+
+Only needed if you enrolled as `pending`. This is the moment a student actually gains entry.
+
+**Cancel an enrollment** (Programs → \[program\] → Enrollments table → Cancel, or Enrollments
+list on a student's own page):
+
+Immediately blocks new Lab launches. Does not touch the student's progress/achievements/attempt
+history — those are permanent regardless of enrollment status. Cancelling is final for that
+enrollment; if the student should have access again later, enroll them again (a new enrollment
+row) rather than trying to "reactivate" a cancelled one.
+
+**Check a student's enrollment history** (Students → \[student\] → "Enrolled programs" card):
+
+Shows every program the student has ever been enrolled in — pending, active, completed,
+cancelled, expired — with progress and last activity. A plain instructor sees this scoped to
+enrollments they created or that belong to a student sharing one of their cohorts; owner/admin
+see everything.
+
+**A student says "Continue Learning" isn't working / shows the wrong thing:**
+
+1. Check their enrollment status on the Programs or Student page. `pending` → activate it.
+   `expired`/`cancelled` → that's working as intended; enroll them again if they should continue.
+2. If `active` and still stuck, check `/api/admin/students/:id/environment` for a stuck Lab —
+   same troubleshooting as any other Lab issue (`docs/RUNBOOK_STUCK_LAB_CLEANUP.md`). Destroying
+   the Lab never loses their program progress; a fresh launch resumes from the same place.
+
+**A student's completion didn't register:**
+
+Completion is detected the next time their program status is *read* (their dashboard, or an
+admin viewing their enrollment) after they finish the capstone — not pushed instantly. Refresh
+the page or reopen their student detail in `/admin`; if it still doesn't show `completed` after
+that, check `GET /api/achievements` for that student for a `capstone_completed` record matching
+the program's configured capstone — if that's missing, the capstone itself wasn't actually
+finished, not an enrollment bug.