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/lemma.typ

@@ -0,0 +1,34 @@
+// lemma: a stated result, optionally with a proof.
+//
+// Contract fields:
+//   content: `stmt` (required), `proof` (optional)
+//   scalar:  none
+//
+// Numbered "定理 N" via the shared lemma-counter.
+//   student: show `stmt`, HIDE `proof`.
+//   teacher: show both (when `proof` is present).
+// `proof` may be absent (key omitted) — handled gracefully.
+
+#import "../fonts.typ"
+#import "common.typ": element-box, tag-line, divider, lemma-counter, hue-of
+
+#let display-lemma(part, show-proof: true) = {
+  let theme = oklch(70%, 0.08, hue-of.lemma * 1deg)
+  lemma-counter.step()
+
+  element-box(hue-of.lemma, {
+    // Statement
+    tag-line(context [定理 #lemma-counter.display()])
+    set text(font: fonts.accent)
+    part.stmt
+
+    // Proof: only when requested AND actually provided.
+    let proof = part.at("proof", default: none)
+    if show-proof and proof != none {
+      divider(theme)
+      tag-line[证明]
+      set text(font: fonts.accent)
+      proof
+    }
+  })
+}