feat: add custom threshold badges

[brandtnewlabs]

Summary

• add LiveChart.renderThresholdBadge for custom React Native threshold badges
• expose live threshold value, formatted value, Y position, visibility, and resolved line config through ThresholdBadgeRenderProps
• preserve the built-in Skia badge when the renderer returns null/undefined, while keeping the dashed marker line native
• document the API and add a realistic average-cost badge to the Threshold example

Verification

• npm run verify — 106 suites passed; 1,596 tests passed, 5 skipped
• React Doctor — 96/100 (two pre-existing maintainability warnings, no errors)
• iOS example app verified with agent-device; recording attached in the PR comments

Closes #291

[brandtnewlabs]

• feat: add custom threshold badges #294 👈 (View in Graphite)
• main

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[brandtnewlabs]

iOS example app proof

The recording shows the built-in threshold pill switching to the custom React Native B/E badge. The chart and formatted value keep updating live, and the badge then moves from the configured left anchor to the right anchor while the native dashed threshold line remains in place.

issue-291-custom-threshold-proof.mp4