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/common.typ

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+// Shared building blocks for element renderers: counters, tag lines, framed
+// boxes, divider lines. Zero external deps (no fontawesome/showybox) — tags use
+// plain bold text labels instead of icon glyphs.
+
+#import "../fonts.typ"
+#import "../style.typ": no-indent
+
+/// Counters for numbered elements. Shared across the package so numbering is
+/// continuous across the lesson regardless of which file renders a part.
+#let example-counter = counter("cph-example")
+#let lemma-counter = counter("cph-lemma")
+
+/// A bold label line introducing a section of an element body, e.g.
+/// "例题 1" or "解析". Starts flush-left (cancels paragraph indent).
+#let tag-line(label) = {
+  no-indent
+  set text(font: fonts.sans, weight: "bold")
+  label
+  h(1em / 3)
+}
+
+/// A faint horizontal divider between sections inside an element box.
+#let divider(color) = block(spacing: 1em, {
+  set align(center)
+  line(length: 100%, stroke: color.darken(20%) + 0.5pt)
+})
+
+/// A framed, tinted, breakable box wrapping an element's content.
+/// `hue` selects the accent color family (degrees on the oklch hue wheel).
+#let element-box(hue, body) = {
+  let theme = oklch(70%, 0.08, hue * 1deg)
+  block(
+    inset: (x: 1.5em, y: 1em),
+    breakable: true,
+    width: 100%,
+    fill: theme.lighten(70%).transparentize(50%),
+    stroke: theme.darken(60%) + 0.8pt,
+    radius: 0.5em,
+    {
+      set par(first-line-indent: 2em, spacing: 1em)
+      body
+      parbreak()
+    },
+  )
+}
+
+/// The accent hue per element kind (keeps the four kinds visually distinct).
+#let hue-of = (
+  example: 250,
+  lemma: 150,
+  segment: 90,
+  sop: 30,
+)

render/src/elements/example.typ

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+// 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
+    }
+  })
+}

render/src/elements/lemma.typ

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+// 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
+    }
+  })
+}

render/src/elements/segment.typ

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+// 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
+  }
+}

render/src/elements/sop.typ

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+// sop: a "standard operating procedure" — a methods/steps block.
+//
+// Contract fields:
+//   content: `sop` (required)
+//   scalar:  none
+//
+// Shown for all targets. Lightly boxed for visual separation; not numbered.
+
+#import "common.typ": element-box, tag-line, hue-of
+
+/// Render a sop part.
+#let display-sop(part, show-sop: true) = {
+  if show-sop {
+    element-box(hue-of.sop, {
+      tag-line[方法]
+      part.sop
+    })
+  }
+}