Improve README and enable DNS resolver by default

Rework top-level README: add badges, status/roadmap section, config
search path docs, minimal two-node example with transports, DNS setup
instructions with systemd-resolved and resolv.conf examples, and
connectivity test walkthrough.

Replace ASCII document-relationship diagram with SVG in design docs.

Change DNS resolver default from disabled to enabled (port 5354).
Update config merge to allow higher-priority configs to disable it.

src/upper/config.rs

@@ -20,11 +20,15 @@ const DEFAULT_DNS_PORT: u16 = 5354;
 /// Default DNS record TTL in seconds (5 minutes).
 const DEFAULT_DNS_TTL: u32 = 300;
 
+fn default_true() -> bool {
+    true
+}
+
 /// DNS responder configuration (`dns.*`).
-#[derive(Debug, Clone, Default, Serialize, Deserialize)]
+#[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct DnsConfig {
-    /// Enable DNS responder (`dns.enabled`).
-    #[serde(default, skip_serializing_if = "std::ops::Not::not")]
+    /// Enable DNS responder (`dns.enabled`, default: true).
+    #[serde(default = "default_true")]
     pub enabled: bool,
 
     /// Bind address (`dns.bind_addr`).
@@ -40,6 +44,17 @@ pub struct DnsConfig {
     pub ttl: Option<u32>,
 }
 
+impl Default for DnsConfig {
+    fn default() -> Self {
+        Self {
+            enabled: true,
+            bind_addr: None,
+            port: None,
+            ttl: None,
+        }
+    }
+}
+
 impl DnsConfig {
     /// Get the bind address (default: 127.0.0.1).
     pub fn bind_addr(&self) -> &str {