docs(phase6b): document curriculum expansion

PHASE6B_ROOT_CURRICULUM_EXPANSION.md — the phase summary: what was
authored per course, the is_required extension, the capstone unlock
fix, verified scenario coverage before/after (44 -> 88 of 98, 89.8%,
by direct query not estimate), and honest known limitations/Phase 6C
recommendations.

ROOT_CAPSTONE_READINESS.md — the readiness module and the server-side
unlock fix in full detail, including why the module's completion and
the real assessment's unlock gate are deliberately independent systems.

Updated ROOT_CURRICULUM_INVENTORY.md (Phase 6B coverage numbers),
ROOT_PROGRAM_ARCHITECTURE.md (ROOT 08/09 PARTIAL, new module lists per
course), COURSE_CONTENT_MODEL.md (is_required completion rule, a new
content-versioning section per this phase's own requirement),
COURSE_AUTHORING_STANDARD.md (classifying near-duplicate/advanced
scenarios), ENGINEERING_SOP.md (new Phase 6B section),
REMOTE_AND_SELF_PACED_ARCHITECTURE.md (cross-reference), and
platform/README.md (e2e-phase6b.js, the Phase 6B rehearsal, isolation-
per-suite regression guidance).

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

docs/ENGINEERING_SOP.md

@@ -461,3 +461,34 @@ Full design: `docs/PHASE6A_ROOT_COURSE_FOUNDATION.md`, plus `docs/ROOT_CURRICULU
   the gold module's real lesson content, launches a real Lab, the Lab's own `setup_commands`
   stand up the broken deployment, a real `kubectl patch` fixes it, and the module completes and
   unlocks the next one) — both against a live stack.
+
+## 18. ROOT_ Curriculum Authoring & Coverage Expansion (Phase 6B)
+
+Full design: `docs/PHASE6B_ROOT_CURRICULUM_EXPANSION.md`, plus
+`docs/ROOT_CURRICULUM_COVERAGE_MATRIX.md` and `docs/ROOT_CAPSTONE_READINESS.md`.
+
+- **What it is:** 13 new teaching modules across ROOT 05-10/12/Capstone
+  (`023_phase6b_curriculum.sql`), built entirely from the 98-scenario inventory Phase 6A already
+  catalogued — no new scenarios needed. ROOT 08/09 moved from PLANNED to PARTIAL; scenario
+  coverage in structured curriculum went from 44/98 to 88/98 (89.8%), verified by direct query,
+  not estimated. 3 modules (plus the pre-existing gold standard) reach the full READY bar; every
+  other new module is honestly PARTIAL, missing a demonstration or guided exercise, documented
+  per-module rather than glossed over.
+- **`is_required`** (new `module_activities` column, defaults `true`): distinguishes CORE
+  teaching from GUIDED/INDEPENDENT-practice duplicates and ADVANCED/optional extras — a module
+  is complete once every REQUIRED activity is done; an incomplete optional one never blocks it.
+  Every Phase 6A activity is unaffected (defaults to required).
+- **The capstone server-side unlock fix:** `POST /api/assessments/:id/start` never checked
+  `unlock_rule` before this phase — a direct API call could start the capstone before its
+  prerequisite stages were complete, even though the UI never exposed that path. Fixed by
+  binding to the same `computeCurriculumState()` gate `GET /api/capstone` already used. See
+  `docs/ROOT_CAPSTONE_READINESS.md` §4 for the full story, including a pre-existing gap this
+  fix exposed in `e2e-phase5b.js`'s own capstone test (fixed alongside).
+- **Tested, not just designed:** `platform/test/e2e-phase6b.js` (24 checks: new-module
+  hierarchy, prerequisite chains through the newly-inserted front-door modules, the
+  optional-vs-required completion rule, the capstone bypass fix in both directions, persistence/
+  isolation/RBAC/draft-hiding proven against the new content) and
+  `infra/scripts/drills/test-phase6b-root-course-rehearsal.js` (11 checks, real k3s: a student
+  walks a brand-new module — ROOT 07 Services & Service Discovery — end to end including a real
+  `kubectl expose` against a live cluster, plus an instructor narrative proving Curriculum view,
+  Prepare Labs, and classroom tooling all still work against the expanded curriculum).