docs(phase6a): document RootNode course foundation

COURSE_AUTHORING_STANDARD.md — the standard every future lesson/module
must follow (target learner, prerequisites, measurable objectives,
concept-before-lab, instructor notes, terminology, versioning, review
requirements), grounded in the gold-standard module as the reference
implementation.

PHASE6A_ROOT_COURSE_FOUNDATION.md — the phase summary: youth-ladder
context, the Google Workspace status already recorded in Phase 5C's own
docs, architecture/build summary, honest content states, test results,
explicit non-goals, and known limitations/Phase 6B recommendations.

Updated ENGINEERING_SOP.md (new Phase 6A section + a curriculum-content
cross-reference), REMOTE_AND_SELF_PACED_ARCHITECTURE.md (a 7th
closed gap: self-paced enrollment now resolves to a real course tree,
not just a scenario list), LAB_ARCHITECTURE.md (Phase 6A reuses the
disposable Lab unmodified, verified by a real rehearsal), and
platform/README.md (course model in the api/ summary, new e2e suite
descriptions for 5A/5B/5C/6A which were missing entirely, and the
real-Lab rehearsal script).

Co-Authored-By: Claude Sonnet 5 <[email protected]>

docs/LAB_ARCHITECTURE.md

@@ -519,3 +519,16 @@ 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.
+
+## Phase 6A: reused, not modified — a teaching layer in front, still one Lab
+
+Phase 6A (`docs/PHASE6A_ROOT_COURSE_FOUNDATION.md`) added a course/module/activity content model
+in front of the exact same disposable Lab this document describes — no change to the
+Provisioner, `environments`, the reaper, the Lab Host Router, or the Lab image itself
+(`Dockerfile`/`entrypoint.sh`/`backend/server.js` untouched). A course's `lab` activity is a
+`scenario_id` reference; "Launch Lab" from the course view calls the identical
+`/api/student/programs/:id/launch` sequence Phase 5B already established — not a third launch
+path. Verified for real, not assumed: `infra/scripts/drills/test-phase6a-course-rehearsal.js`
+launches a real Lab, calls the Lab's own `POST /api/scenarios/:id/setup` through the Gateway's
+existing `/lab/:token/*` proxy exactly as the browser UI does, and runs a real `kubectl patch`
+against the live k3s cluster inside that container before recording completion.