[Bug]: any markdown link crashes the macOS renderer (empty array stored as underlineStyle) #128

Summary

any markdown link crashes the macOS/AppKit renderer with NSInvalidArgumentException: -[Swift.__EmptyArrayStorage integerValue]: unrecognized selector. root cause: Markdown.Link.convert (Sources/MarkdownText/Inline/Markdown+InlineConvertible.swift, the container[.underlineStyle] = [] line) stores an empty Swift array as the .underlineStyle attribute value — in a [NSAttributedString.Key: Any] context the [] literal infers as an empty array, not as NSUnderlineStyle. TextKit expects an NSNumber there and AppKit calls -integerValue on it during layout, crashing NSTextView. the UIKit path tolerates the bad value, which is why iOS never crashed.

Minimal markdown input

a [link](https://example.com) is enough

Platform

macOS (the AppKit paragraph rendering layer; iOS is unaffected)

Expected behavior

the link renders styled and not underlined, as on iOS.

Actual behavior

the sample app crashes as soon as a document containing a link is rendered, e.g. opening the Kitchen Sink demo:

Thread 1: EXC_BREAKPOINT
FAULT: NSInvalidArgumentException: -[Swift.__EmptyArrayStorage integerValue]: unrecognized selector sent to instance

Validation

• found while testing on the macOS sample app: opening the Kitchen Sink demo (full of links) crashes immediately; demos without links render fine.
• iOS simulator renders the same fixtures correctly.
• CI stays green because no snapshot/unit test currently renders a plain link on macOS (citation tests replace links with attachments).
• fix is a one-liner (NSUnderlineStyle([]).rawValue) — PR with a regression test incoming.