Draft changelog: Aug 29 X 动态 API and usage updates

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Summary

Adds a weekly changelog entry (Aug 29, 2026) covering user-facing X API v2 changes shipped since the Aug 13 entry.

Changes

• 新建 features: broadcast.start/broadcast.end events, filter.qualifiers on X 动态 API subscriptions, and the GET /2/usage/credits endpoint
• Updates: pay-per-use monthly Post read cap raised from 2M to 3M, is_subscriber field on broadcast.chat payloads
• Docs-only merges (nav fixes, broken links, doc corrections) were excluded

Context

Based on the 12 PRs merged to xdevplatform/docs since the last changelog entry (Aug 13, 2026).

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Project	Status	Preview	Updated (UTC)
x-corp	🟢 Ready	View Preview	Aug 29, 2026, 8:08 PM

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