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/bad-target-config/manifest.toml

@@ -5,12 +5,14 @@ name = "bad-target-config"
 [info]
 title = "测试课：非法导出目标配置"
 
-# Invalid artifact value → SchemaViolation, falls back to default (single-file).
+# Invalid artifact `type` → SchemaViolation, falls back to default (single-file).
 [targets.student]
-artifact = "pdf-thing"
+artifact = { type = "pdf-thing", filepath = "build/student.pdf" }
 
-# numbering.heading is not an array of strings → SchemaViolation, heading None.
+# Invalid step `type` → SchemaViolation; the bad step is skipped and the build
+# falls back to the default step.
 [targets.teacher]
-artifact = "file-tree"
-[targets.teacher.numbering]
-heading = "not-an-array"
+artifact = { type = "file-tree", root = "build/teacher", outputs = "**/*.html" }
+[[targets.teacher.steps]]
+type = "make-it-nice"
+template = "exports/teacher.typ"