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.

requirements-native.txt

@@ -0,0 +1,38 @@
+#
+# Runtime dependencies for the GraalPy native standalone executable
+# (Dockerfile.native). Unlike requirements.txt this deliberately excludes
+# granian, rloop and kaya-rsgi: the standalone binary serves the app over ASGI
+# with Hypercorn, because those compiled extensions cannot be bundled into a
+# native executable.
+#
+--index-url https://gitea.woggioni.net/api/packages/woggioni/pypi/simple
+--extra-index-url https://pypi.org/simple
+
+hypercorn==0.18.0
+    # via pyfconfig (native)
+h11==0.16.0
+    # via hypercorn
+h2==4.4.1
+    # via hypercorn
+hpack==4.2.0
+    # via h2
+hyperframe==6.1.0
+    # via h2
+priority==2.0.0
+    # via hypercorn
+wsproto==1.3.2
+    # via hypercorn
+kaya-core==0.0.3
+    # via
+    #   kaya-forwarded
+    #   pyfconfig (pyproject.toml)
+kaya-forwarded==0.0.3
+    # via pyfconfig (pyproject.toml)
+pwo==0.1.2
+    # via
+    #   kaya-core
+    #   pyfconfig (pyproject.toml)
+typing-extensions==4.16.0
+    # via
+    #   kaya-core
+    #   pwo