docs: add cross-chain announcement format spec

[Peolite1]

Closes #90

This PR introduces the normative specification for cross-chain announcements emitted by Wraith Protocol.

Changes Included:

• New Architecture File: Added architecture/announcement-format.mdx.
• Common Canonical Form: Documented the standardized JSON format used across the SDK.
• Per-Chain Specifications: Included the event topics and payload shapes for currently supported chains (CKB, EVM, Solana, Stellar).
• Extensibility Notes: Documented guidelines for integrating future chains into the SDK.

Acceptance Criteria Met:

• Every chain in sdk/chains/* is documented.
• The specification is marked as normative.
• Ready for review by the SDK maintainer.

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[truthixify]

In, thanks. Normative spec is what third-party indexers were waiting for.