feat(spec+checker): slides/逐字稿 markdown 内容面 + assembleMarkdown step(ADR-0015)

给课程加两个 markdown 内容面:slides(大纲:h2 part → h3 小节 → ![]() 图 +
:::widget 教具)与逐字稿(口播)。直接 markdown+KaTeX 撰写,绕开 typst→md
公式转换(等于在 slides 面选 ADR-0014 的 R2)。

spec:
- RichContent.lean: content leaf 带 format(typst|markdown),新增 ContentFormat
  归纳,RichContentRef 加 format 字段
- Export/Render.lean: Step 加 assembleMarkdown (field),钉执行语义
  (同 shell 三边界 + 注入课程 h1 + 自包含收集引用图)
- Courseware.lean: ADR 区间 →0015

实现:
- cph-schema: x-cph-content 支持 true→.typ / 字符串→该扩展名(.md);
  ContentField 带 ext;4 个 kind schema 各加可选 slides/transcript
- cph-model: Step::AssembleMarkdown{field} + manifest "assemble-markdown" 解析
- cph-check: run_markdown_assemble_target / target_is_markdown_assemble
  (照 run_shell_target 形状:check 门控、注入 h1、按 [[parts]] 序拼、写盘后
  收集 ![](rel) 引用图进 build 根使产物自包含、引用图缺失属 build-过程错误
  不入 6 类诊断);compile_targets 已 filter TypstCompile,纯 assemble 自然排除
- cph-cli: run_markdown_assemble_build 路由
- cph-typst: template_step 补 AssembleMarkdown => None 分支

测试:全 workspace 49 passed(新增 6 个 markdown 装配测试含图收集);
lake build 25 jobs 绿。

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

crates/cph-model/src/lib.rs

@@ -163,6 +163,7 @@ impl Artifact {
 /// inductive Step where
 ///   | typstCompile (template : String)
 ///   | shell (run : String)
+///   | assembleMarkdown (field : String)
 /// ```
 ///
 /// A step is a *typed* operation (extensible): `TypstCompile` compiles a
@@ -170,9 +171,13 @@ impl Artifact {
 /// the manifest into it (`--input manifest=…`), which a bare `typst compile`
 /// string cannot express. `Shell` is the escape hatch for steps that resist
 /// declaration (ADR-0005's medium-only category (b): HTML interactive / npm
-/// builds). The on-disk `[[steps]]` entry's `type` discriminator selects the
-/// variant: `"typst-compile"` → [`Step::TypstCompile`], `"shell"` →
-/// [`Step::Shell`].
+/// builds). `AssembleMarkdown` concatenates a per-element markdown content
+/// field into a single-file markdown deliverable (ADR-0015: the slides outline
+/// and 逐字稿 transcript surfaces, authored directly in markdown+KaTeX to
+/// sidestep the typst→md conversion). The on-disk `[[steps]]` entry's `type`
+/// discriminator selects the variant: `"typst-compile"` →
+/// [`Step::TypstCompile`], `"shell"` → [`Step::Shell`], `"assemble-markdown"`
+/// → [`Step::AssembleMarkdown`].
 ///
 /// As with [`Artifact`], this alignment is maintained by review, not CI.
 #[derive(Debug, Clone, PartialEq, Eq, Serialize)]
@@ -189,6 +194,15 @@ pub enum Step {
         /// The command line to run.
         run: String,
     },
+    /// Assemble a single-file markdown deliverable by concatenating each
+    /// element's `field` markdown content file in `[[parts]]` order. Mirrors
+    /// Lean `Step.assembleMarkdown` (ADR-0015). Not a typst build — the
+    /// framework owns the read/concatenate/write itself.
+    AssembleMarkdown {
+        /// The per-element markdown content field to assemble (e.g. `slides`,
+        /// `transcript`).
+        field: String,
+    },
 }
 
 impl Step {
@@ -901,7 +915,9 @@ fn parse_step(name: &str, value: toml::Value, diags: &mut Vec<Diagnostic>) -> Op
                     DiagCode::SchemaViolation,
                     format!("target '{name}' has a step with a non-string `type`"),
                 )
-                .with_hint("set the step `type` to \"typst-compile\" or \"shell\""),
+                .with_hint(
+                    "set the step `type` to \"typst-compile\", \"shell\", or \"assemble-markdown\"",
+                ),
             );
             return None;
         }
@@ -911,7 +927,9 @@ fn parse_step(name: &str, value: toml::Value, diags: &mut Vec<Diagnostic>) -> Op
                     DiagCode::SchemaViolation,
                     format!("target '{name}' has a step with no `type` discriminator"),
                 )
-                .with_hint("add `type = \"typst-compile\"` (or `\"shell\"`) to the step"),
+                .with_hint(
+                    "add `type = \"typst-compile\"` (or `\"shell\"`, `\"assemble-markdown\"`) to the step",
+                ),
             );
             return None;
         }
@@ -948,16 +966,33 @@ fn parse_step(name: &str, value: toml::Value, diags: &mut Vec<Diagnostic>) -> Op
                 None
             }
         },
+        "assemble-markdown" => match take_string_field(&mut table, "field") {
+            Some(field) => Some(Step::AssembleMarkdown { field }),
+            None => {
+                diags.push(
+                    Diagnostic::error(
+                        DiagCode::SchemaViolation,
+                        format!(
+                            "target '{name}' assemble-markdown step is missing a string `field`"
+                        ),
+                    )
+                    .with_hint("add `field = \"slides\"` (or `\"transcript\"`) to the step"),
+                );
+                None
+            }
+        },
         other => {
             diags.push(
                 Diagnostic::error(
                     DiagCode::SchemaViolation,
                     format!(
                         "target '{name}' has a step with an invalid `type` '{other}'; \
-                         valid values are \"typst-compile\", \"shell\""
+                         valid values are \"typst-compile\", \"shell\", \"assemble-markdown\""
                     ),
                 )
-                .with_hint("set the step `type` to \"typst-compile\" or \"shell\""),
+                .with_hint(
+                    "set the step `type` to \"typst-compile\", \"shell\", or \"assemble-markdown\"",
+                ),
             );
             None
         }