Improve axis and crosshair overlays

[Federicocervelli]

Summary

• keep X/Y axis endpoint labels readable and hide overlapping tick labels
• add generic X-range metadata for picked samples
• let crosshair highlights render X-range rectangles by default and expose a custom renderHighlight hook
• add crosshair labelPlacement option

Tests

• bun run typecheck
• bun run build
• bun run docs:readme
• bun run test:generated-docs
• bun run test:visual

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Export the new X-range dataset types

These new interfaces are the only named contract for datasets that want crosshair/tooltip interval highlighting, but they are not re-exported from either src/index.ts or src/core/index.ts, so package consumers cannot import XRangeDataset/XRange through the public blazeplot or blazeplot/core entry points despite the runtime now checking for getXRange(). This makes the new extension point effectively internal for TypeScript users; add these types to the public barrels alongside the other dataset capability interfaces.

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