[codex] Prepare 6.12.2 release

[ghost]

Summary

• bump package version to 6.12.2
• document the chart edge-case fixes in the changelog
• update the publish workflow to create a GitHub Release/tag after npm publish
• verify both the npm package version and git tag are unused before publishing

Why

The current workflow failed because react-native-chart-kit@6.12.1 is already published. It also only publishes to npm, so a successful workflow run does not appear in GitHub Releases.

Validation

• npm run typecheck
• npm run build
• npm test -- --watch=false --passWithNoTests
• npm_config_cache=/tmp/react-native-chart-kit-npm-cache npm pack --dry-run
• npm view react-native-chart-kit@6.12.2 version returns 404, confirming 6.12.2 is unpublished
• remote tag v6.12.2 does not exist