feat(checker): cph-typst — embed typst 0.15, driver-gen, compile, PDF (WU-4)

The heart of the pipeline. Embeds the typst 0.15 compiler as a library to
compile-check a lesson and export a PDF, with diagnostics mapped back to source
spans (the product's value).

- LessonWorld: a typst World over the engineering-file directory tree. vpath =
  real relative path (ADR-0007); the per-target driver is mounted in-memory as
  `main` (.cph/driver-<target>.typ, never written to disk); @local/cph-render is
  resolved from render_dir (CPH_RENDER_DIR / with_render_dir), no package install
  or network. Fonts via typst-kit FontStore (system CJK).
- Driver generation: from the manifest, per target — static root-relative
  `include` of each content file (⇒ module body content, ADR-0006), assembled
  into the frozen `display(info, target, parts)` call; optional lemma proof
  emitted only if proof.typ exists; example `source` scalar threaded through.
- Diagnostics: typst SourceDiagnostic (errors + warnings) → cph_diag::Diagnostic.
  0.15 DiagSpan handled via span.id() + WorldExt::range → file-relative
  SourceSpan with 1-based line/col; driver-internal spans re-pointed with a
  "generated driver" hint, never dropped. typst hints/trace carried through.
- Engine API: new / with_render_dir / compile_check / build_pdf.
- 6 tests incl. build_pdf_with_real_render — full World→driver→cph-render→PDF
  producing real PDF-1.7 output (student 42KB / teacher 54KB). clippy/fmt clean.

Also fixes render/typst.toml `compiler = ">=0.15.0"` → `"0.15.0"`: typst 0.15's
manifest parser rejects a `>=` comparator on the compiler field (the 0.14 CLU
WU-2 tested against was lenient). This unblocks loading the real render package.

Pinned: typst/typst-pdf/typst-syntax/typst-library/typst-layout/typst-kit =0.15.0.

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

render/typst.toml

@@ -2,7 +2,7 @@
 name = "cph-render"
 version = "0.1.0"
 entrypoint = "lib.typ"
-compiler = ">=0.15.0"
+compiler = "0.15.0"
 authors = ["curriculum-project-hub"]
 license = "MIT"
 description = "Curriculum lesson render package: single `display` entry over an ordered list of typed parts (segment/example/lemma/sop), targeting student/teacher handouts."