chore: align ARCP wire protocol version with v1.1

Bumps Version.PROTOCOL and Envelope.VERSION to "1.1" and updates the
round-trip test, wire-format reference, and README feature line.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: nficano

README.md

@@ -131,7 +131,7 @@ export JAVA_HOME=$(/usr/libexec/java_home -v 21)
 
 ## Features
 
-- §5.1 envelope with `arcp: "1"` and `FAIL_ON_UNKNOWN_PROPERTIES=false`
+- §5.1 envelope with `arcp: "1.1"` and `FAIL_ON_UNKNOWN_PROPERTIES=false`
 - §6.1 bearer auth via `BearerVerifier` SPI; `acceptAny` and `staticToken` helpers
 - §6.2 capability intersection with rich `agents` shape (name + versions + default)
 - §6.3 resume buffer (in-memory ring) and rotating `resume_token`

arcp-core/src/main/java/dev/arcp/core/Version.java

@@ -1,7 +1,7 @@
 package dev.arcp.core;
 
 public final class Version {
-    public static final String PROTOCOL = "1";
+    public static final String PROTOCOL = "1.1";
     public static final String SDK = "1.0.0";
 
     private Version() {}

arcp-core/src/main/java/dev/arcp/core/wire/Envelope.java

@@ -29,7 +29,7 @@ public record Envelope(
         @JsonProperty("event_seq") @Nullable Long eventSeq,
         JsonNode payload) {
 
-    public static final String VERSION = "1";
+    public static final String VERSION = "1.1";
 
     public Envelope {
         Objects.requireNonNull(arcp, "arcp");

arcp-core/src/test/java/dev/arcp/core/EnvelopeRoundTripTest.java

@@ -35,7 +35,7 @@ void envelopeWithSessionIdRoundTrips() throws Exception {
         String json = mapper.writeValueAsString(original);
         Envelope parsed = mapper.readValue(json, Envelope.class);
 
-        assertThat(parsed.arcp()).isEqualTo("1");
+        assertThat(parsed.arcp()).isEqualTo("1.1");
         assertThat(parsed.id()).isEqualTo(MessageId.of("m_1"));
         assertThat(parsed.sessionId()).isEqualTo(sid);
         assertThat(parsed.type()).isEqualTo("session.hello");

docs/05-reference/wire-format.md

@@ -13,7 +13,7 @@ since: "1.0.0"
 
 ```json
 {
-  "arcp": "1",
+  "arcp": "1.1",
   "id": "01J…",
   "type": "session.hello",
   "session_id": "sess_…",
@@ -26,7 +26,7 @@ since: "1.0.0"
 
 | Field | Java type | Always required |
 |---|---|---|
-| `arcp` | `String` | yes (`Envelope.VERSION = "1"`) |
+| `arcp` | `String` | yes (`Envelope.VERSION = "1.1"`) |
 | `id` | [`MessageId`](../../arcp-core/src/main/java/dev/arcp/core/ids/MessageId.java) | yes (ULID, monotonic per process) |
 | `type` | discriminator → [`Message.Type`](../../arcp-core/src/main/java/dev/arcp/core/messages/Message.java) | yes |
 | `payload` | `JsonNode` then decoded per type | yes |