Returns the change in a player's stats since the start of the period: the latest snapshot diffed against the most recent snapshot at or before the period's reset. The delta is a recursive diff: unchanged fields are omitted, a changed numeric stat is the bare difference (new minus old, e.g. 50 or -3), and a field that appeared, disappeared, or changed non-numerically is { "old": <previous or null>, "new": <current or null> }, with old null for a stat absent from the baseline snapshot. A player not snapshotted near the period's reset (e.g. one returning after a long absence) is diffed against an older, possibly partial snapshot, which can surface lifetime totals as { "old": null, "new": <total> }.