Add self-contained GraalPy native executable

Build the app to a single native binary with graalpy's 'standalone
native' command (Dockerfile.native). granian/rloop cannot be bundled
into a standalone binary, so the executable serves the app over ASGI
with Hypercorn via native/entrypoint.py, which also works around
GraalPy standalone quirks (socket.listen, str response bodies, signal
handling).

Adds docker-compose.native.yml, a -native CI job, and README docs.

.dockerignore

@@ -16,7 +16,9 @@ build/
 tests/
 Dockerfile
 Dockerfile.graalpy
+Dockerfile.native
 .dockerignore
 opencode.json
 docker-compose.yml
 docker-compose.graalpy.yml
+docker-compose.native.yml