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.

docker-compose.native.yml

@@ -0,0 +1,24 @@
+# Native standalone variant of the app. Standalone file (not an override) so
+# it can be run alongside the other compose files:
+#
+#   docker compose -f docker-compose.native.yml up -d --build
+#
+# The app is compiled to a single self-contained GraalPy native executable; see
+# Dockerfile.native.
+services:
+  pyfconfig-native:
+    build:
+      context: .
+      dockerfile: Dockerfile.native
+    image: pyfconfig-native:latest
+    restart: unless-stopped
+    environment:
+      # Public name shown in the HTML page and the curl examples; set to the
+      # deployment domain (e.g. "ifconfig.example.com").
+      SITE_NAME: ${SITE_NAME:-pyfconfig}
+      # Comma-separated CIDRs of trusted reverse proxies (e.g.
+      # "172.16.0.0/12"); required for correct client IPs when deployed
+      # behind a reverse proxy. Empty means no proxy is trusted.
+      TRUSTED_PROXY_CIDRS: ${TRUSTED_PROXY_CIDRS:-}
+    ports:
+      - "127.0.0.1:8080:8080"