Initial boilerplate scaffold for continuum-schemas

Protobuf data model (Template/PrintJob/MaterialProfile/PrintParameters)
with codegen targets for TypeBox+TS types (packages/ts-types) and
prost-derived Rust structs (packages/rust-types).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.env.example

@@ -0,0 +1,8 @@
+# continuum-schemas has no runtime — it only emits generated types/structs.
+# This file is kept for consistency with the other Continuum repos.
+
+# Optional: private registry token, only needed if packages/* are ever
+# published to a private npm/cargo registry instead of consumed as
+# workspace/path dependencies.
+NPM_REGISTRY_TOKEN=
+CARGO_REGISTRY_TOKEN=