Implement Spencer's 15-Point Moving Average

[Copilot]

This PR adds Spencer's 15-Point Moving Average (SMA15), a specialized weighted moving average designed for optimal data smoothing while preserving trend characteristics.

Implementation Details

Spencer's 15-Point Moving Average uses a fixed 15-point window with specific weights:

[-3, -6, -5, 3, 21, 46, 67, 74, 67, 46, 21, 3, -5, -6, -3]

These weights sum to 320, which is used as the denominator for the weighted average calculation.

Changes

• Added src/SMA15/SMA15.ts with both standard and faster implementations
• Added comprehensive tests in src/SMA15/SMA15.test.ts
• Updated exports in index.ts
• Added entry in README.md

Usage

import {SMA15} from 'trading-signals';

const sma15 = new SMA15();

// Add values to fill the 15-point window
for (let i = 0; i < 15; i++) {
  sma15.add(i);
}

// Get the Spencer's 15-point weighted average
const result = sma15.getResult();

Unlike other moving averages in the library that take a customizable interval parameter, Spencer's 15-Point Moving Average always uses 15 points with the predefined weights.

Fixes #807.

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