fix: pre-v0.1 review feedback (#14)

* fix(sdk-android): add @JvmOverloads to Uatu.start

Java callers can now invoke start(application) without supplying a
Configuration, matching the Kotlin default-arg ergonomics.

* feat(agent): add protocol_version to HELLO handshake

ProtocolVersion=1 lives on Message and is set by Hello(). Server.Accept
rejects mismatches with a clear error. SDK upgrades that don't change
the wire format keep the same protocol_version; bump on breaking changes.

* test(agent): assert protocol_version in Hello round-trip

* feat(sdk-android): send protocol_version=1 in HELLO

Mirrors agent.ProtocolVersion on the Go side. Bump in lockstep with
the Go constant when the wire format breaks.

* chore(sample-app): pull @uatu/spec from npm next tag

Replaces the file: dep. Copy-paste users can now npm install against
the registry. The release workflow publishes pre-release tags to
npm dist-tag 'next', so the sample tracks the latest rc without
manual version bumps. Lockfile currently resolves to 0.0.1-rc3.

internal/agent/protocol.go

@@ -21,13 +21,18 @@ const (
 
 const MaxFrameSize = 16 * 1024 * 1024
 
+// ProtocolVersion is the wire-format version. Bump on any breaking change
+// to the message schema or framing. Independent of the SDK release version.
+const ProtocolVersion = 1
+
 type Message struct {
 	Type MessageType `json:"type"`
 	ID   uint64      `json:"id,omitempty"`
 
-	Version    string `json:"version,omitempty"`
-	Platform   string `json:"platform,omitempty"`
-	AppPackage string `json:"app_package,omitempty"`
+	ProtocolVersion int    `json:"protocol_version,omitempty"`
+	Version         string `json:"version,omitempty"`
+	Platform        string `json:"platform,omitempty"`
+	AppPackage      string `json:"app_package,omitempty"`
 
 	Snapshots map[string]json.RawMessage `json:"snapshots,omitempty"`
 
@@ -39,7 +44,13 @@ type Message struct {
 }
 
 func Hello(version, platform, appPackage string) Message {
-	return Message{Type: MessageTypeHello, Version: version, Platform: platform, AppPackage: appPackage}
+	return Message{
+		Type:            MessageTypeHello,
+		ProtocolVersion: ProtocolVersion,
+		Version:         version,
+		Platform:        platform,
+		AppPackage:      appPackage,
+	}
 }
 
 func Pause(id uint64) Message { return Message{Type: MessageTypePause, ID: id} }