feat(feeEstimator): add estimateFeeForAmount helper with bigint precision (Fixes #615)

[s6pa1rta3n-lab]

Fixes #615

Summary of Changes

• Added estimateFeeForAmount(amount: bigint, feeStats: FeeStats): { feeLumens: bigint; feePercent: number; totalWithFee: bigint } exported from src/feeEstimator.ts.
• Derived feeLumens from feeStats.baseFee while retaining bigint precision throughout, computing totalWithFee = amount + feeLumens.
• Computed feePercent as Number(feeLumens) / Number(amount) * 100 and safely returned 0 when amount === 0n.
• Added validation throwing SdkError with code INVALID_RECIPIENT (SdkErrorCode.INVALID_RECIPIENT) if amount is negative.
• Added and exported FeeStats interface ({ baseFee: bigint; p50Fee: bigint; p99Fee: bigint }) in src/types.ts.
• Exported estimateFeeForAmount, estimateFee, estimateOperationCost, FeeStats, and fee estimator types from src/index.ts.
• Added unit tests in test/feeEstimator.test.ts covering known amounts, zero amount calculation, bigint preservation, and negative amount validation.

Acceptance Criteria Checklist

• 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 } — added to src/types.ts
• Exported from src/index.ts
• Unit tests: correct fee calculation for known amount/stats, zero amount returns 0 percent, negative amount throws

Payout Routing

• EVM (Base/Arbitrum/Polygon/ETH): 0xF46C9F6d70C50BF81ef3588AB523a90a594a2F89
• Stellar: GCL6OXAMLD75BMTINA6EMRUDWK5THQUSHMYNLSNBCJAPZJHNYJTUNIBC