Validate that split amounts sum to exactly 100% before execution in splitExecutor

[Kingsman-99]

Description

splitExecutor processes split payment instructions without verifying that recipient percentages sum to 100. Floating-point rounding errors or user input mistakes silently produce splits that over- or underpay the total.

Acceptance Criteria

• Before building any transaction, the executor checks that the sum of all recipient ratios equals 1.0 within a tolerance of 1e-9
• A SplitRatioSumError is thrown with the actual sum when the check fails
• Valid splits (sum within tolerance) proceed unchanged
• Unit tests pass

Context

• Target file: src/payments/splitExecutor.ts
• The tolerance 1e-9 should be exported as SPLIT_RATIO_TOLERANCE