docs: lock v1 scope and terminology

FEAT.md

@@ -10,6 +10,33 @@ This repository now targets the split-plugin architecture only.
 
 ---
 
+## Shared Terminology and v1 Scope
+
+Canonical terms used across the project:
+- `identifier`: unique logical client name
+- `rule_identifier`: exact-match application route key
+- `builtin`: reserved protocol/system route namespace
+- `pairingCode`: short-lived out-of-band pairing code
+- `secret`: shared secret issued after pairing
+- `publicKey` / `privateKey`: client signing keypair
+
+Locked v1 decisions:
+- `heartbeat_ack` is optional
+- client reconnect uses exponential backoff
+- rule matching is exact-match only
+- offline sends fail immediately instead of queueing
+- `mainHost` is expected to be a full `ws://` or `wss://` URL
+
+Explicit v1 non-goals:
+- multi-server topology
+- direct client-to-client sockets
+- offline queueing guarantees
+- advanced rule matching
+- management UI
+- admin approval control plane beyond human relay of pairing codes
+
+---
+
 ## 1. Yonexus.Server Features
 
 ### 1.1 Server Runtime