feat(render): export templates + render-lesson entry; template owns include loop (WU-C', ADR-0011)

Replace the display(parts) entry (which a generated driver called) with the
template model: real editable templates (render/templates/{student,teacher}.typ,
the framework defaults copied into an engineering file's exports/) read the
manifest via toml(sys.inputs.manifest) and assemble parts themselves.

Key finding (drives the engine): a dynamic `include` inside a package resolves
against the PACKAGE root, never --root — so the include loop must live in the
TEMPLATE (which sits under --root): `include "/" + part.path + "/" + field +
".typ"` (root-relative absolute, runtime-computed — legal per ADR-0011's verified
fact). cph-render exposes render-lesson(info, target, parts, heading-numbering)
(content-in, never includes) + part-fields (kind→fields) + default-heading-numbering.
Numbering (numbly per-level) is set in the template, editable per engineering file.

Surfaced OPEN point closed by the engine (WU-D'): typst has no file-exists
primitive, so optional content (lemma proof) presence comes from a per-part
`fields` array the engine computes from disk. Manifest injected as root-relative
absolute (--input manifest=/...). numbly stays vendored/offline. display removed.

local-packages symlink (test-only @local resolution) gitignored — regenerable,
self-referential; the embedded engine mounts cph-render directly.

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

render/templates/README.md

@@ -0,0 +1,63 @@
+# Default export templates (ADR-0011)
+
+`student.typ` / `teacher.typ` are the **framework default templates**. In a real
+engineering file they live at `exports/student.typ` / `exports/teacher.typ`; the
+project-creation flow seeds copies there (out of scope here — see ADR-0011 Open
+Questions). They are kept here as the canonical, copyable defaults and for the
+offline smoke test below.
+
+Each template:
+
+1. reads the injected manifest: `toml(sys.inputs.manifest)`;
+2. loops `manifest.parts`, `include`-ing each content field via a computed
+   **root-relative absolute** path `/<part.path>/<field>.typ`, and reading scalar
+   fields (example `source`) from `/<part.path>/element.toml`;
+3. assembles a `parts` array and calls `cph-render`'s `render-lesson(...)`,
+   passing the per-level heading numbering (presentation lives in the template).
+
+## Why the include loop is in the template, not in cph-render
+
+typst resolves a dynamic `include`/`toml` path relative to **the file the call
+lexically appears in**, and a package (`@local/cph-render`) has its **own virtual
+root**. A dynamic `include` written inside the package resolves against the
+*package* dir — even an absolute `/segments/x.typ` — never the engineering
+`--root`. Verified empirically. The template lives under `--root`, so its
+`/<part.path>/<field>.typ` resolves against `--root`. Hence the template loads
+content and hands `render-lesson` an already-assembled `parts` array;
+`render-lesson` never includes anything.
+
+## Key path facts for the engine
+
+- **Template is the typst main file.** Compile it directly.
+- **`--root` = engineering-file root.** Content `include`s use absolute
+  root-relative paths.
+- **Manifest is injected as a root-relative ABSOLUTE path**:
+  `--input manifest=/manifest.toml`. `toml(sys.inputs.manifest)` resolves
+  relative to the *template's* location (`exports/`), so a bare
+  `manifest=manifest.toml` would look in `exports/`. Pass the leading `/`.
+- **Optional content presence** (lemma `proof`): typst has no file-exists
+  primitive, so the template cannot probe disk. It reads a per-part `fields`
+  array from the manifest listing the content fields present on disk. *(OPEN: the
+  exact manifest shape for this is for the Rust/manifest contract to pin.)*
+
+## Offline smoke test
+
+`@local/cph-render` is resolved from a local-packages dir (a symlink to the
+render package root — regenerable, test-only):
+
+```sh
+cd render
+# one-time: make @local/cph-render resolvable offline
+mkdir -p vendor/local-packages/local/cph-render
+ln -sfn "$(pwd)" vendor/local-packages/local/cph-render/0.1.0
+
+# compile a template against the smoke engineering file
+typst compile --root examples/smoke-eng \
+  --package-path ./vendor/local-packages \
+  --package-cache-path ./vendor/typst-packages \
+  --input manifest=/manifest.toml \
+  examples/smoke-eng/exports/student.typ /tmp/cph-tmpl-student.pdf
+```
+
+(`examples/smoke-eng/exports/{student,teacher}.typ` are copies of the defaults
+here, mirroring how a real engineering file carries its own templates.)