feat(spec): add lesson model contract (ADR-0005)

First product-core concept in the semantic master spec: a curriculum
engineering file is one lesson, modeled as an ordered sequence of typed
element instances over an open kind universe.

- docs/adr/0005: lesson model decision narrative + deferred OPEN items
- spec/Spec/Courseware/Lesson.lean: Primitives carrier (KindId/ElementData/
  TargetId/RenderRule kept abstract), Element, Lesson, RenderConfig, and the
  first checker seed (WarnsIgnored: used-kind-with-no-render-rule => warning)
- wire Spec.lean -> Spec.Courseware; drop placeholder Basic.lean

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

spec/Spec/Courseware.lean

@@ -0,0 +1,9 @@
+import Spec.Courseware.Lesson
+
+/-!
+# Courseware —— 产品层契约(课程工程文件)
+
+本层是护城河:课程"工程文件"的语义母本与合法性规则。当前只有 lesson 骨架
+(见 `Spec.Courseware.Lesson`,ADR-0005);题库、course 编排、完整合法性判定
+等仍 `OPEN`,待各自 ADR 落定后逐个加入。
+-/