Add moving average calculation for fee trend analysis in fees/trend.ts

[Kingsman-99]

Description

fees/trend.ts collects fee samples over time but only exposes raw data and no derived statistics. A moving average would smooth noise and give callers a reliable signal for fee forecasting.

Acceptance Criteria

• computeMovingAverage(samples: number[], windowSize: number): number[] is exported
• The function returns a Simple Moving Average series of the same length, with the first windowSize - 1 entries padded as NaN
• windowSize must be at least 1; values less than 1 throw a RangeError
• Unit tests pass

Context

• Target file: src/fees/trend.ts
• The function must be pure (no side effects, no state mutation)