feat(checker): compile template as main, augmented manifest closes optional-content (WU-D', ADR-0011)

Replace generated-driver compilation with the template model.

cph-typst:
- driver.rs DELETED (generate_driver, static-include driver, numbering-threading).
- New manifest.rs: build_augmented_manifest(&Lesson) emits a TOML doc with [info]
  + ordered [[parts]] each carrying a `fields` array = the kind's content fields
  whose <root>/<path>/<field>.typ exists on disk. Reuses cph-schema's
  content_field_names (new dep; no cycle). This closes the OPEN point WU-C'
  surfaced: typst has no file-exists primitive, so the engine (which has disk
  access) tells the template which optional content (lemma proof) is present.
- World main = the real engineering-file template (<root>/<template>); the
  augmented manifest is served as an in-memory virtual file at /.cph/manifest.toml
  (never written to the tree), injected via sys.inputs.manifest. render_dir kept
  only for @local/cph-render + vendored @preview/numbly resolution.
- Artifact handling: SingleFile+TypstCompile compiles; FileTree, shell-only, and
  undeclared-target are blocking "deferred/not-declared" diagnostics (ADR-0011).
- Engine public signatures unchanged; LessonWorld::new takes template+manifest_src.

cph-check: no source change needed (only uses lesson.targets/target.name +
unchanged Engine methods); pipeline + Legal alignment intact.

Fixtures: mini gets exports/{student,teacher}.typ (from render/templates/) + v2
manifest + a second proof-less lemma to exercise optional content. render-stub
retired (tests use the real render/). Through-template PDFs offline: student 44KB,
teacher 56KB; proof-less lemma compiles clean (optional-content confirmed).
Workspace: fmt + clippy -D warnings + all tests green.

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

Cargo.lock

@@ -436,6 +436,7 @@ version = "0.1.0"
 dependencies = [
  "cph-diag",
  "cph-model",
+ "cph-schema",
  "tempfile",
  "toml",
  "typst",