fix: add missing return statements in mergeSingleEvent()

The method declared a return type of Track but had two code paths
with missing returns: the early return when no events exist returned
undefined, and the end of the function had no return at all. Both
now return this to match the type contract and enable method chaining.

Closes #147

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: grimmdude

src/chunks/track.ts

@@ -184,7 +184,7 @@ class Track implements Chunk {
 		// There are no events yet, so just add it in.
 		if (!this.events.length) {
 			this.addEvent(event);
-			return;
+			return this;
 		}
 
 		// Find index of existing event we need to follow with
@@ -220,6 +220,8 @@ class Track implements Chunk {
 				this.events[i].delta = this.events[i].tick - this.events[i - 1].tick;
 			}
 		}
+
+		return this;
 	}
 
 	/**