Token-Gated Invoice Access Controller

[Kingsman-99]

Description

Some invoice workflows should be readable only by accounts holding a minimum balance of a specific Stellar asset (e.g. a governance or membership token). The SDK has no access control primitive tied to on-chain token ownership. A token-gated access controller should verify a caller's token balance before returning invoice data, integrating with Server.loadAccount() to read balances on demand without requiring off-chain authentication.

Technical Context

Add src/tokenGateController.ts. Reads BalanceLineAsset from @stellar/stellar-sdk account data. Accepts TokenGatePolicy (asset, minimum balance) added to src/types.ts. Integrates with src/client.ts getInvoice() and src/accessControl.ts. Throws TokenGateAccessDeniedError from src/errors.ts when the caller's balance is below the minimum. Caches balance checks via src/cache.ts with a short TTL.

Acceptance Criteria

• TokenGateController.verify(callerAccountId: string, policy: TokenGatePolicy): Promise<void> resolves when the caller meets the balance requirement and throws TokenGateAccessDeniedError otherwise
• Balance check result is cached for cacheTtlMs (configurable, default 15 000 ms) to reduce Horizon calls
• src/client.ts getInvoice() calls TokenGateController.verify() when invoice.accessPolicy is set
• TokenGatePolicy supports { asset: Asset, minBalance: string, strict: boolean } where strict: false only warns without blocking
• Unit tests mock loadAccount() for two scenarios: balance above minimum (resolves) and balance below minimum (throws)
• All CI checks (npm test, npm run build, ESLint/TypeScript) pass and the branch has no merge conflicts

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