chore: align ARCP wire protocol version with v1.1

Bumps Version.Protocol to "1.1" and updates codec/envelope round-trip
tests + the README envelope description.

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

Author: Nick Ficano

README.md

@@ -168,7 +168,7 @@ The SDK is organised as eight projects (`Arcp.Core`, `Arcp.Client`,
 `Arcp.Runtime`, `Arcp.AspNetCore`, `Arcp.Giraffe`, `Arcp.Otel`,
 `Arcp`, `Arcp.Cli`). The high-level shape:
 
-- **Wire envelope**: 8 fields; `arcp = "1"`; `payload : JsonElement` for
+- **Wire envelope**: 8 fields; `arcp = "1.1"`; `payload : JsonElement` for
   lazy decode; codec uses `FSharp.SystemTextJson` with
   `JsonUnionEncoding.InternalTag` keyed on `type` so the discriminator
   sits at the same level as peer fields.

src/Arcp.Core/Version.fs

@@ -5,7 +5,7 @@ namespace ARCP.Core
 module Version =
     /// Wire-level protocol version emitted as the envelope `arcp` field.
     [<Literal>]
-    let Protocol = "1"
+    let Protocol = "1.1"
 
     /// SDK package version. Pinned alongside `Directory.Build.props`.
     [<Literal>]

tests/Arcp.UnitTests/CodecTests.fs

@@ -51,7 +51,7 @@ let ``job.event with progress body roundtrips`` () =
 [<Fact>]
 let ``unknown message type returns InvalidRequest`` () =
     let env = {
-        Arcp = "1"
+        Arcp = "1.1"
         Id = "x"
         Type = "nonsense.type"
         SessionId = None

tests/Arcp.UnitTests/EnvelopeTests.fs

@@ -26,8 +26,8 @@ let ``envelope roundtrip preserves wire fields`` () =
     parsed.EventSeq |> should equal (Some 7L)
 
 [<Fact>]
-let ``envelope arcp field is "1" not "1.0"`` () =
+let ``envelope arcp field is "1.1"`` () =
     let env = Envelope.create "session.hello" (Json.parseElement "{}")
     let wire = Codec.writeEnvelope env
-    wire |> should haveSubstring "\"arcp\":\"1\""
-    wire |> should not' (haveSubstring "\"1.0\"")
+    wire |> should haveSubstring "\"arcp\":\"1.1\""
+    wire |> should not' (haveSubstring "\"arcp\":\"1.0\"")