feat(client): net primitives — DmResponse, NarrateResult, DmTransport, ProxyConfig

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

client/scripts/net/dm_transport.gd

@@ -0,0 +1,10 @@
+class_name DmTransport
+extends RefCounted
+## The network seam. DmService talks to this, never to HTTPRequest directly, so
+## the loop logic is testable with an injected fake. Concrete transports
+## override post_json. The base returns a failed response (rather than assert())
+## so a missing override is a loud, catchable failure in tests.
+
+func post_json(_path: String, _body: Dictionary) -> DmResponse:
+	push_error("DmTransport.post_json must be overridden")
+	return DmResponse.failed("transport not implemented")