Dotted and dashed underline support for annotated links

[bjdodson-openai]

Summary

Adds first-class link decoration support to compose-richtext so consumers can render dotted/dashed underlines that respect line wrapping, and wires the API through Markdown rendering.
Updates the demo to showcase the new styles and line wrapping.

Changes

• New API: RichTextDecorations, LinkDecoration, and UnderlineStyle to customize link underline styles.
• Link rendering now suppresses default underline when a custom underline is applied.
• Underline rendering uses TextLayoutResult to draw line-aware dotted/dashed underlines.
• Markdown entrypoints and renderers pass richTextDecorations through.
• Demo updated with dotted/dashed link examples and a long, wrapping dotted link.
• AstBlockNodeComposer.Compose signature updated to include richTextDecorations.

Behavior

• Links matched by LinkDecoration.matcher can render dotted/dashed underlines while preserving default behavior for other links.
• Underlines follow line wraps and align visually with text layout.

Testing

Notes / Compatibility

• Any custom AstBlockNodeComposer implementations must add the new richTextDecorations parameter.

Follow-ups

• Add inline image override API to compose-richtext for parity with app-side reflection logic.
• Consider consolidating underline styles into a CustomDash if we want a single style type.

Codex

Prompt/context: “Add reusable dotted/dashed underline support for links in compose-richtext; draw underlines using TextLayoutResult so they respect wrapping. Wire through Markdown and update the demo to show long wrapped dotted underline examples.”