Draft changelog: Activity API qualifiers, usage credits, PPU cap

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Summary

Adds an Aug 30, 2026 changelog entry covering product changes shipped since the last entry (Aug 13).

Changes

• New features: filter.qualifiers for X Activity API subscriptions, broadcast.start/broadcast.end events, and the GET /2/usage/credits endpoint.
• Updates: Pay-per-use monthly Post read cap raised from 2M to 3M, and the is_subscriber field on broadcast.chat event payloads.
• Docs-only merges (nav fixes, sample corrections, directory updates) were excluded.

Context

Sourced from PRs merged to xdevplatform/docs between Aug 13 and Aug 30, 2026.

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Project	Status	Preview	Updated (UTC)
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