feat(transform): add native financial indicator dataset transform

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What problem does this feature solve?

Currently, developers building financial/candlestick charts must rely on external JS libraries to calculate technical indicators before passing them to ECharts. This PR introduces a native echarts:indicator transform that computes these mathematically in the data pipeline, significantly reducing boilerplate and external dependencies.

What does the proposed API look like?

The new transform hooks seamlessly into the existing ExternalDataTransform interface:

dataset: [{
    source: stockData
}, {
    transform: {
        type: 'echarts:indicator',
        config: { 
            indicator: 'macd', // Supports: 'sma', 'ema', 'macd', 'bollinger'
            shortPeriod: 12, 
            longPeriod: 26, 
            signalPeriod: 9, 
            sourceDimension: 'Close' 
        }
    }
}]

[echarts-bot]

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