Add estimateFeeForAmount with bigint precision to feeEstimator

[Kingsman-99]

Description

src/feeEstimator.ts estimates transaction fees but does not expose a helper that takes a payment amount and returns the estimated fee as a percentage and absolute value. Callers must read raw fee stats and do the math themselves.

Acceptance Criteria

• estimateFeeForAmount(amount: bigint, feeStats: FeeStats): { feeLumens: bigint; feePercent: number; totalWithFee: bigint } exported from src/feeEstimator.ts
• feeLumens is derived from feeStats.baseFee; totalWithFee = amount + feeLumens
• feePercent is Number(feeLumens) / Number(amount) * 100 — returns 0 when amount === 0n
• Throws SdkError with code INVALID_RECIPIENT if amount is negative
• FeeStats type: { baseFee: bigint; p50Fee: bigint; p99Fee: bigint } — add to src/types.ts if not present
• Exported from src/index.ts
• Unit tests: correct fee calculation for known amount/stats, zero amount returns 0 percent, negative amount throws

Context

• Target files: src/feeEstimator.ts, src/types.ts, src/index.ts
• feeLumens must remain bigint throughout — only convert to number for feePercent

[bobo-24]

Hi! I’m a full-stack developer with a strong blockchain background, and I’m confident I can resolve this issue quickly and effectively. I’ll deliver a clean, reliable solution with immediate attention to detail, ensuring it’s completed as soon as possible. Please assign it to me—I’m ready to get started right away.

[grantfox-oss]

🦊 GrantFox — @bobo-24 has been assigned to this issue as part of the Third Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #615)
2. Your PR will be reviewed by the Stellar-split maintainers

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