Draft changelog: streaming latency improvements

[mintlify]

Summary

Adds a Sep 3, 2026 changelog entry covering the streaming latency improvement shipped this week.

Changes

• New entry: X API v2 volume streaming endpoints (e.g. Filtered Stream) now deliver at ~4-5 seconds P99 latency, down from ~6-7 seconds.
• Links to the Filtered Stream, Powerstream, and streaming fundamentals docs.

Context

Reviewed everything merged to main since the Sep 1 entry. The only user-facing product change was the latency improvement reflected in the streaming docs; the remaining commits were automated OpenAPI spec syncs with no functional API changes.

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