Add histogram series support

[Federicocervelli]

Summary

• add pure histogram binning helpers and HistogramDataset interval metadata
• add Chart.addHistogram and declarative createChart histogram series
• add docs, website example, unit tests, and visual-test coverage

Validation

• bun run typecheck
• bun test
• bun run docs:readme
• bun run test:generated-docs
• bun run test:docs-snippets
• bun run pages:build
• bun run test:visual -- --cases histogram
• bun run test:exports
• bun run test:package
• bun run test:bundle-size

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Limit histogram zero-baseline fit to histogram axis

createChart(...) now forces fitToData({ includeZero: true }) whenever any histogram series is present, but includeZero is applied to both Y axes inside Chart.fitToData. In mixed-axis charts (e.g., histogram on left axis plus a right-axis series with values far from zero), the right axis is unnecessarily expanded to include 0, which can severely compress the non-histogram data and distort the default first render. This regression is triggered by default autoFit: true and should be scoped to histogram series/axis only (or left to explicit user opt-in).

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