fix: Added a warning when tracks are being added before player init

[spuppo-mux]

Closes https://github.com/muxinc/devextravaganza/issues/215

Relates to a behavior we often see (mainly on Safari) that adding a track to the player element before it initializes, causes playback to buffer indefinitely. For example https://github.com/muxinc/devextravaganza/issues/215.

While this PR does not fix the issue, it adds a warning so this behavior is easier to identify.

An example where this could happen is if on React a user were to write this it would log a warning:

const Player = (props) => {
    return (<MuxPlayer
        {...props}
        ref={(instance) => {
            instance?.addChapters(props.chapters);
            props.ref.current = instance;
        }
    />)
}

To address this they should do something like this:

const Player = (props) => {
    return (<MuxPlayer
        {...props}
        ref={(instance) => {
            if (instance) {
                const handler = () => instance.addChapters(muxChapters ?? []);
                instance.addEventListener(
                    "loadedmetadata", 
                    () => instance.addChapters(props.chapters), 
                    { once: true }
                );
            }
            
            props.ref.current = instance;
        }
    />)
}

---

Note

Low Risk
Low risk: adds console warnings only; behavior and data flow for chapters/cue points remain unchanged aside from extra logging that could be noisy in some apps.

Overview
Adds a guard in MuxVideoBaseElement.addCuePoints() and addChapters() to console.warn when these are called before the media element has a currentSrc, nudging callers to wait for initial load (e.g., loadstart). No functional change to how cue points/chapters are added beyond emitting the warning.

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