feat(render): rebuild typst render package cph-render (WU-2)

Single entry `display(info, target, parts)` over an ordered parts array, the
frozen contract the generated typst driver calls. Per-kind dispatch for
segment/example/lemma/sop; student/teacher matrix derived internally from
`target` (student hides example solution + lemma proof; teacher shows all).
Content-field values are consumed as already-evaluated content (driver
`include`s them ⇒ module body, ADR-0006), never imported/stringified. Optional
fields (lemma proof, example source) handled gracefully; unknown kind/target
degrade without crashing. Zero @preview deps (CI-robust); CJK + math styled.
Smoke compiles to non-empty student/teacher PDFs.

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

render/src/elements/segment.typ

@@ -0,0 +1,16 @@
+// segment: a stretch of textbook prose.
+//
+// Contract fields:
+//   content: `textbook` (required)
+//   scalar:  none
+//
+// Rendered identically for all targets (textbook is always public). Not boxed
+// or numbered — it is the connective body text of the lesson.
+
+/// Render a segment part. `show-textbook` is always true in MVP but kept for
+/// symmetry with the other kinds' show/hide flags.
+#let display-segment(part, show-textbook: true) = {
+  if show-textbook {
+    part.textbook
+  }
+}