ref(core): Use primitive long for EventProcessorAndOrder.order (#5527)

* ref(core): Use primitive long for EventProcessorAndOrder.order

Avoid boxing by storing the order as a primitive long instead of a
boxed Long. The constructor already normalizes a null order to
System.nanoTime(), so the field never needs to represent null.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* changelog

* changelog

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Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>