Draft changelog: X Activity events, qualifiers, usage credits

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Summary

Adds a Sep 2, 2026 changelog entry covering user-facing changes shipped since the last entry.

Changes

• New: broadcast.start / broadcast.end public events for the X Activity API
• New: filter.qualifiers on X Activity API subscriptions (conversation_type, direction)
• New: GET /2/usage/credits for checking Pay-per-use credit balance
• Update: Pay-per-use monthly Post read cap raised from 2M to 3M
• Update: is_subscriber boolean on broadcast.chat event payloads

Docs-only merges (navigation fixes, sample corrections, link fixes) were excluded.

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Project	Status	Preview	Updated (UTC)
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