Sponsorship Reserve Verifier

[Kingsman-99]

Description

The SDK's sponsorship feature (via src/sponsorship.ts) builds BeginSponsoringFutureReserves / EndSponsoringFutureReserves operation pairs but does not verify in advance that the sponsoring account has sufficient XLM reserve to cover the new ledger entries it will sponsor. This causes submission failures that are cryptic to callers. A pre-submission reserve verifier should calculate the required reserve increase and check the sponsor's available balance.

Technical Context

Extend src/sponsorship.ts and src/preflightChecker.ts. Uses @stellar/stellar-sdk Server.loadAccount() to read subentry_count and num_sponsored, and the Stellar network's BASE_RESERVE constant. Reference SponsorshipConfig from src/types.ts. Add InsufficientSponsorReserveError to src/errors.ts. Expose the check as a method callable before the sponsorship transaction is built in src/txBuilder.ts.

Acceptance Criteria

• verifySponsorReserve(sponsorId: string, newEntries: number): Promise<ReserveCheckResult> calculates required XLM and compares with available balance
• Returns { sufficient: boolean, required: string, available: string, shortfall?: string } typed in src/types.ts
• Throws InsufficientSponsorReserveError with the shortfall amount when the check fails and throwOnInsufficient: true
• src/sponsorship.ts calls verifySponsorReserve() before building the sponsorship operation pair
• Unit tests mock loadAccount() responses and assert correct reserve arithmetic for 0, 1, and 10 new entries
• All CI checks (npm test, npm run build, ESLint/TypeScript) pass and the branch has no merge conflicts

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[GdAyo19]

@GdAyo19 has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

I would love to work on this issue

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[adelekevictor12]

@adelekevictor12 has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

Hello maintainer, I can fix this issue

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[lekescrew22]

@lekescrew22 has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

i would love to work on this issue

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[drips-wave]

Congratulations, @lekescrew22! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on July 30, 2026.

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