Function recordLoadEvent

Apply a single load event to the metrics counters in-place.

Updates:

• loads: incremented by 1
• elementsLoaded: incremented by elements (points / vertices / splats)
• bytesLoaded: incremented by bytes
• avgLoadTime: rolling mean (prev_avg × (n-1) + new_load_time) / n after the increment

Pure-ish: mutates counters (the only side-effect) and returns it.