Draft changelog: X Activity API updates and PPU credits endpoint

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Summary

Adds a Sep 2, 2026 changelog entry covering product changes shipped since the last changelog entry.

Changes

• New feature: filter qualifiers (filter.qualifiers) on X Activity API subscriptions
• New feature: broadcast.start and broadcast.end events for the X Activity API
• New feature: GET /2/usage/credits endpoint for Pay-Per-Use credit balance
• Update: is_subscriber field on broadcast.chat event payloads

Context

Sourced from merged PRs #342, #324, #329, and #340. Docs-only changes (navigation fixes, page fixes) were excluded.

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