Formal `ChainScanner` adapter contract for third-party chains

[truthixify]

Tier: M (2-4 days) | Type: refactor

Context. scanAll (PR #149) hard-wires the four supported chains via SupportedChain = 'evm' | 'stellar' | 'solana' | 'ckb' in src/scanner/unified.ts. To let downstream projects add a chain without patching the SDK, formalise the contract each chain adapter must satisfy and expose it as a public interface. This is the v1.0 extensibility story we punted on in PR #152.

Scope.

• Extract a ChainScannerAdapter<TAnnouncement, TMatch> interface: { id, scan(source, keys), decodeMetaAddress, encodeMetaAddress }.
• Refactor each src/chains/*/scan.ts to also export an adapter object satisfying the interface.
• scanAll accepts either the built-in SupportedChain shorthand or an arbitrary adapter.
• Api-extractor snapshot updated; conformance test loops over registered adapters.

Acceptance.

• Interface documented in the reference with a runnable third-party example under examples/
• All four in-tree chains implement the interface
• scanAll({ adapters: [customChain] }) compiles and runs against a fixture
• etc/sdk.api.md reviewed and updated

Files. src/scanner/unified.ts, src/chains/*/scan.ts, src/index.ts, etc/sdk.api.md, examples/custom-chain-adapter/ (new).

[Good-terminal]

@Good-terminal has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hi maintainer I will like to work on this issue please

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[BALLUCK004]

@BALLUCK004 has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hello Maintainer, This aligns with my experience in [tech/stack]. I’ll deliver a PR that addresses the issue, includes tests, and follows the project’s coding standards. Can I get assigned?

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[Jerryvic911]

@Jerryvic911 has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hi, I’d like to work on this. I have experience with TypeScript, testing, and developer tooling, and I’m comfortable improving error handling and debugging utilities. I can implement the describe() helper with clear, actionable fix hints and cover the changes with tests.

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