feat: add authenticated realtime presence

ARCHITECTURE.md

@@ -118,6 +118,16 @@ adapters whose absence cannot stop the city, office, traffic, or sky from
 running. This is the replacement path for the retired Phaser World 1; it is not
 a migration of that renderer.
 
+The first hosted adapter is now concrete: the Fastify API owns short-lived,
+in-memory authoritative presence sessions and streams validated deltas over
+authenticated POST/SSE. `src/realtime` remains renderer- and transport-neutral;
+the browser transport, interpolation buffer, procedural peer renderer and small
+status UI are separate adapters. A city or full-depth office opts into remote
+peers explicitly, while an anonymous/public scene does not even allocate their
+assets. Interest changes clear the old coordinate frame before a new one is
+shown. Game-state deltas never carry webcam or screen media; those remain
+separate consent and authorization lifecycles.
+
 ---
 
 ## 3. Three rules that keep Apache 2.0 honest