Bug: mergeSingleEvent() missing return statements

[grimmdude]

Description

mergeSingleEvent() in src/chunks/track.ts declares a return type of Track but has two code paths with missing/incorrect returns:

1. Line 175 — early return has no value (return; instead of return this;)
2. End of function (~line 210) — no return statement at all

mergeSingleEvent(event: AbstractEvent): Track {
    if (!this.events.length) {
        this.addEvent(event);
        return;  // returns undefined, not this
    }
    // ... rest of function ...
    // no return statement
}

Impact

Violates the Track return type contract. Any code attempting to chain off mergeSingleEvent() would get undefined.

Suggested Fix

Add return this; to both code paths.