Carry the theme's link decoration through to the screen

[paulscan]

Setting MarkdownEditorTheme.link has no visible effect. NSTextView layers its linkTextAttributes over every .link range at display time, on top of whatever foreground the styler set. AppKit's default paints them with .linkColor (system blue). A custom link color gets computed, emitted into [StyledRange], applied to the text storage, and then silently overridden on screen.

An embedder that sets theme.link = .white on a dark canvas still sees a blue link. This makes building color themes difficult.

What's fixed

MarkdownEditorTheme now exposes a computed linkTextAttributes. It carries the theme's link color, a configurable underline style, and the same pointing-hand cursor AppKit's default carries. NativeTextViewWrapper assigns it in makeNSView and refreshes it in updateNSView alongside the existing insertionPointColor refresh.

The theme also controls the underline through MarkdownEditorTheme.linkUnderlineStyle, which defaults to .single. That matches AppKit's stock look. Setting it to [] drops the underline for palettes where it reads as clutter, such as a monospace canvas or a poster-style page.

Passing a theme built with defaults leaves the rendered result pixel-identical to before.

Tests

Tests/MarkdownEngineTests/LinkTextAttributesTests.swift covers:

• The default theme's attributes match AppKit's stock link look:
  .linkColor, .single underline, pointing-hand cursor.
• A custom link color reaches the attributes.
• Setting linkUnderlineStyle to [] or .thick propagates
  through.

swift build and swift test are green.