API reference: `fetchAnnouncementsStream` deep-dive

[truthixify]

Extra labels: writing, api-reference
Tier: M (2-4 days) | Type: writing

Context. Options object has grown (retention, view-tag filter, cache). API reference is out of date. Deep-dive that covers every option + edge case.

Scope.

• Expand api-reference/fetch-announcements-stream.mdx.
• Every option's type + default + example.
• Error taxonomy (RetentionExceededError et al).
• Cancellation semantics.
• Backpressure notes.

Acceptance.

• Matches current SDK exports (grep verification)
• All examples pass CI
• Every option covered

Files. api-reference/fetch-announcements-stream.mdx.