docs(v4.0.0): talking nodes -- a node is born empty and takes in its first capsule

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/v4.0.0/ENGINE.md

@@ -316,6 +316,15 @@ Rulings on that step so far:
   spin; it is blocked reading its ports and wakes when a neighbour writes,
   and the console handing it a line is a message arriving on a port like
   any other.
+- **A node is born empty (ruled, "A").** It has nothing but the ability
+  to listen: it is blocked reading its ports, and the first thing a
+  neighbour sends it is a capsule of F18 code, which it takes in and runs,
+  as an F18 node executes what arrives at its port. StarForth's nucleus is
+  then itself a capsule — the one that makes an empty node able to digest
+  FORTH-source capsules — and is named, hashed, signed and recorded in
+  parity like any other, where today it is linked into the binary. The one
+  thing every node has, whatever it becomes, is the fixed behaviour at
+  reset by which it takes in its first capsule.
 - **How it grows: "Only the central node can connect to only another
   central node."** So the five are a unit: four outer nodes and their
   centre. An outer node is wired only inside its own unit. Units are joined