Architecture: cross-chain announcement format spec

[truthixify]

Extra labels: writing, architecture, spec
Tier: M (2-4 days) | Type: writing

Context. Every chain emits announcements slightly differently. Third-party indexers need a normative spec they can code against.

Scope.

• New architecture/announcement-format.mdx.
• Per-chain event topic + payload shape.
• Common canonical form (JSON) used by SDK.
• Extensibility notes for future chains.

Acceptance.

• Every chain in sdk/src/chains/* documented
• Marked as normative
• Reviewed by SDK maintainer

Files. architecture/announcement-format.mdx (new).