feat(checker): align cph-typst + cph-check to export-as-build model (WU-D)

Integrate the WU-B (structured targets) and WU-C (numbly, config) changes.

cph-typst:
- World resolves any @preview/* package from the in-repo vendored dir
  (<render_dir>/vendor/typst-packages/preview/<name>/<version>/), fully offline —
  no typst-kit download. Proven: build_pdf_with_real_render compiles the real
  render package (which imports @preview/numbly:0.1.0) through the embedded
  engine, PDF out.
- Driver threads each target's presentation config: looks up the TargetConfig by
  name, emits `config: (numbering: (heading: (...)))` (escaped) when an override
  is present, else `config: (:)` (render default).
- target_precheck guards artifact/target: FileTree → blocking diagnostic (MVP is
  single-file, deferred per ADR-0009); --target not declared → blocking
  diagnostic; no-targets lesson still defaults through.

cph-check:
- Migrated to Lesson.targets: Vec<TargetConfig> (target_names() / target.name at
  the 6 sites). Pipeline unchanged (already matches ADR-0010 phases). Documented
  that !has_errors() implements Spec.Courseware.Legal (no error-level diagnostic).

Workspace: fmt + clippy -D warnings + all tests green.

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

crates/cph-typst/tests/fixtures/render-stub/lib.typ

@@ -4,7 +4,10 @@
 // It exercises the same call shape as the real entry: it reads `info`, walks
 // `parts` IN ORDER, and renders each part's content fields by `kind`. Content
 // values are already-evaluated content (the driver produced them via include).
-#let display(info: (:), target: "student", parts: ()) = {
+//
+// `config` mirrors the real entry's ADR-0009 presentation-config parameter; the
+// stub accepts it (the driver always passes `config:`) but ignores it.
+#let display(info: (:), target: "student", parts: (), config: (:)) = {
   set document(title: info.at("title", default: ""))
   [= #info.at("title", default: "")]
   [target: #target]