feat: add defensive queue to _sendEventStream to prevent event loss

[crisryantan]

Summary

• Adds an eventStreamQueue buffer in _sendEventStream so that events are queued (instead of silently dropped) when window.Rokt.__event_stream__ is not yet available
• Queued events are flushed in FIFO order once __event_stream__ becomes available on the next processEvent call
• This is a defensive measure to close the gap where events could be lost if the Rokt WSDK hasn't fully initialized when the kit forwards them

Test plan

• Updated existing tests ("should not throw when __event_stream__ is not defined" and "should not throw when window.Rokt is undefined") to verify events are queued rather than dropped
• Added test: "should flush queued events in FIFO order when __event_stream__ becomes available"
• Verify all 128 tests pass in CI

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