Draft changelog: X Activity API qualifiers, broadcast events, PPU credits

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Summary

Adds a weekly changelog entry (Aug 29, 2026) covering product changes shipped since the last entry on Aug 13.

Changes

• New feature: optional filter.qualifiers map on X Activity API subscriptions (conversation_type, direction)
• New feature: broadcast.start and broadcast.end public events in the X Activity API
• New feature: GET /2/usage/credits endpoint for checking Pay-Per-Use credit balance
• Update: is_subscriber boolean on broadcast.chat event payloads

Context

Sourced from PRs merged to xdevplatform/docs since Aug 13 (#324, #329, #340, #342). Docs-only fixes (navigation, broken links, stale reference pages) were excluded since they are not product changes.

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Project	Status	Preview	Updated (UTC)
x-corp	🟢 Ready	View Preview	Aug 29, 2026, 2:14 AM

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