feat(checker): cph-model manifest v2 — Artifact-with-fields + typed steps (WU-B', ADR-0011)

[targets.*] reworked: artifact is now an inline table with a type discriminator
({type="single-file",filepath} / {type="file-tree",root,outputs}) and steps is an
ordered array-of-tables ({type="typst-compile",template} / {type="shell",run}).
numbering removed entirely (presentation lives in the template per ADR-0011).

TargetConfig { name, artifact: Artifact, steps: Vec<Step> }; Artifact and Step
mirror Spec.Courseware.Artifact / Spec.Courseware.Step (doc-cited). Sensible
defaults so a bare [targets.student] still works: single-file build/<name>.pdf +
one typst-compile exports/<name>.typ. Malformed artifact/step type → non-fatal
SchemaViolation with fallback. 8 tests green.

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

crates/cph-model/tests/fixtures/target-configs/manifest.toml

@@ -5,15 +5,24 @@ name = "target-configs"
 [info]
 title = "测试课：导出目标的结构化 build 配置"
 
-# Full config: explicit artifact + numbering.heading override.
+# Full v2 config: explicit single-file artifact + an ordered typst-compile step.
 [targets.student]
-artifact = "single-file"
-[targets.student.numbering]
-heading = ["{1:一}、", "{1:1}.{2:1}", "{1:1}.{2:1}.{3:1}"]
+artifact = { type = "single-file", filepath = "build/student.pdf" }
+[[targets.student.steps]]
+type = "typst-compile"
+template = "exports/student.typ"
 
-# file-tree artifact, no presentation override.
+# file-tree artifact + multiple steps (typst-compile then a shell step),
+# exercising both step types and order preservation.
 [targets.archive]
-artifact = "file-tree"
+artifact = { type = "file-tree", root = "build/archive", outputs = "**/*.{html,js,json}" }
+[[targets.archive.steps]]
+type = "typst-compile"
+template = "exports/archive.typ"
+[[targets.archive.steps]]
+type = "shell"
+run = "npm run build"
 
-# Empty body → all defaults.
+# Empty body → all defaults (single-file build/teacher.pdf, one typst-compile
+# step exports/teacher.typ).
 [targets.teacher]