refactor: streamline markdown rendering by introducing RenderedSegment and MarkdownSegmentRenderer

[hryhoriiK97]

What/Why?

Extracts the segment rendering logic (RenderSegment types and renderTextSegment) from EnrichedMarkdown.kt into a shared RenderedSegment.kt in the common module. This eliminates duplicated text-segment rendering code between EnrichedMarkdown and MeasurementStore, ensuring both consumers use the same MarkdownSegmentRenderer.render() path. Pure structural refactor with no behavioral changes.

Testing

PR Checklist

• Code compiles and runs on iOS
• Code compiles and runs on Android
• Updated documentation/README if applicable
• Ran example app to verify changes