Add some utility methods to tags

book/05-events.md

@@ -100,17 +100,38 @@ other events, pubkeys, topics, and whatever else. Tags and content travel
 together because tags are part of what the hash commits to — they are not
 metadata in the "could be added later" sense.
 
+`EventContent` is a builder. `new` hands back an empty payload and
+chainable `content` and `tags` setters fill it in. Neither field is
+required at construction time — a zero-length note with no tags is a
+perfectly legal kind-1 event — and the builder shape means callers
+don't have to pass `""` or `vec![]` placeholders when they only care
+about one of the two.
+
 ```rust {file=coracle-lib/src/events.rs}
 /// The content of an event: the human-readable body plus its tags.
-#[derive(Debug, Clone, PartialEq, Eq)]
+#[derive(Debug, Clone, PartialEq, Eq, Default)]
 pub struct EventContent {
     pub content: String,
     pub tags: Tags,
 }
 
 impl EventContent {
-    pub fn new(content: impl Into<String>, tags: impl Into<Tags>) -> Self {
-        EventContent { content: content.into(), tags: tags.into() }
+    /// An empty payload: no content, no tags.
+    pub fn new() -> Self {
+        Self::default()
+    }
+
+    /// Set the human-readable content. Returns `self` so the call can
+    /// chain.
+    pub fn content(mut self, content: impl Into<String>) -> Self {
+        self.content = content.into();
+        self
+    }
+
+    /// Set the tags. Returns `self` so the call can chain.
+    pub fn tags(mut self, tags: impl Into<Tags>) -> Self {
+        self.tags = tags.into();
+        self
     }
 }
 ```
@@ -324,7 +345,8 @@ With that in place, the full pipeline from "hello nostr" to a signed event is
 one expression:
 
 ```rust
-let hashed = EventContent::new("hello nostr", vec![])
+let hashed = EventContent::new()
+    .content("hello nostr")
     .kind(1)
     .stamp(1_700_000_000)
     .own(secret.public_key())