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/example.typ

@@ -0,0 +1,38 @@
+// example: a worked example / problem.
+//
+// Contract fields:
+//   content: `problem` (required), `solution` (required)
+//   scalar:  `source` (optional string)
+//
+// Numbered "例题 N" via the shared example-counter.
+//   student: show `problem`, HIDE `solution`.
+//   teacher: show both.
+// `source`, when present, is shown as a small annotation on the problem line.
+
+#import "../fonts.typ"
+#import "common.typ": element-box, tag-line, divider, example-counter, hue-of
+
+#let display-example(part, show-solution: true) = {
+  let theme = oklch(70%, 0.08, hue-of.example * 1deg)
+  example-counter.step()
+
+  element-box(hue-of.example, {
+    // Problem
+    tag-line(context [例题 #example-counter.display()])
+    let source = part.at("source", default: none)
+    if source != none and source != "" {
+      text(size: 0.85em, fill: gray.darken(30%))[（来源：#source）]
+      h(0.3em)
+    }
+    set text(font: fonts.accent)
+    part.problem
+
+    // Solution (teacher-only by default)
+    if show-solution {
+      divider(theme)
+      tag-line[解析]
+      set text(font: fonts.accent)
+      part.solution
+    }
+  })
+}