Build Stellar TOML metadata parser and on-chain anchor verifier

[Kingsman-99]

Description

Anchor-issued assets used in StellarSplit invoices must have a valid stellar.toml file published at the asset issuer's home domain, but the SDK does not validate this before accepting custom assets. This issue adds a StellarTomlParser that fetches and parses stellar.toml files according to SEP-1, and an AnchorVerifier that cross-references the TOML metadata with on-chain account data to confirm the issuer account's home_domain points back to the TOML file (bidirectional verification).

Technical Context

• New file: src/anchors/StellarTomlParser.ts and src/anchors/AnchorVerifier.ts; parsed result type: TomlMetadata mirrors the SEP-1 top-level fields
• StellarTomlParser.fetch(domain: string): Promise<TomlMetadata> uses fetch() to retrieve https://{domain}/.well-known/stellar.toml and parses it with a lightweight TOML parser (@iarna/toml or equivalent)
• AnchorVerifier.verify(assetIssuer: string, assetCode: string): Promise<VerificationResult> performs: (1) load issuer account from Horizon to get home_domain, (2) fetch TOML from home_domain, (3) assert TOML CURRENCIES array contains an entry matching assetCode and issuer
• VerificationResult: { verified: boolean; tomlUrl: string; currencyEntry?: TomlCurrency; issues: string[] }
• TOML responses are cached for tomlCacheTtlMs (default 3 600 000 ms) per domain

Acceptance Criteria

• fetch('example.com') retrieves and correctly parses all SEP-1 TOML sections (ACCOUNTS, CURRENCIES, VALIDATORS, DOCUMENTATION)
• verify('GABC...', 'USDC') returns { verified: true } when the issuer's home_domain TOML contains a matching CURRENCIES entry
• verify() returns { verified: false, issues: ['no_home_domain'] } when the issuer account has no home_domain set
• verify() returns { verified: false, issues: ['currency_not_found'] } when the TOML exists but lacks the assetCode entry
• TOML is served from cache on a second fetch() call within TTL; after TTL, a fresh HTTP request is made
• All CI checks (npm test, npm run build, ESLint) pass and the branch has no merge conflicts

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