feat: add markdownLinkRenderer hook

[ajram23]

Summary

Adds markdownLinkRenderer, mirroring the existing markdownImageRenderer pattern so consumers can inject a custom SwiftUI view for inline markdown links — e.g. favicon-prefixed links, hover tooltips, scheme-specific link UI.

Motivation

MarkdownImageRenderer already lets consumers customize image rendering per URL scheme. Links have no equivalent — every consumer that wants custom link UI (favicons, tooltips, internal scheme handling) has to wrap or fork. This PR fills that symmetric gap with the same API shape.

API

struct MyLinkRenderer: MarkdownLinkRenderer {
    func makeBody(configuration: Configuration) -> some View {
        HStack { faviconFor(configuration.url); configuration.label }
    }
}

MarkdownView(content)
    .markdownLinkRenderer(MyLinkRenderer(), forURLScheme: "https")
    // or, with the "*" wildcard, for ALL schemes:
    .markdownLinkRenderer(MyLinkRenderer())

Configuration exposes url: URL and label: AnyView. Dispatch is by URL scheme with "*" as a wildcard — extends the image-renderer's exact+lowercased-scheme match to also support a catch-all.

Design notes

Env-scoped storage, not a singleton. Renderers live on MarkdownRendererConfiguration.linkRenderers: [String: AnyMarkdownLinkRenderer] and participate in Equatable so CmarkFirstMarkdownViewRenderer's view cache invalidates correctly when a renderer instance changes. The modifier writes via transformEnvironment. No global state. Two callsites registering different renderers for the same scheme stay isolated to their respective subtrees.

(Note: the same global-singleton anti-pattern exists in MarkdownImageRenders upstream. Intentionally only fixing the link side here to minimize divergence — happy to follow up with a matching image-side fix in a separate PR if you'd like.)

Sendable requirement on the protocol. Mirrors SwiftUI's AnyShape pattern (AnyShape requires S: Sendable on init). Required for Swift 6 region-based isolation because AnyMarkdownLinkRenderer's init captures the generic renderer: D into three stored closures.

⚠️ Breaking change for consumers: every type conforming to MarkdownLinkRenderer must now also conform to Sendable. Free for types with no stored properties — synthesis is implicit. New API, so the breakage only affects anyone tracking this branch pre-merge.

Configuration itself is intentionally NOT Sendable because AnyView's Sendable conformance is @available(*, unavailable) per Apple. The protocol is @preconcurrency @MainActor, so the config never crosses an actor boundary in practice.

Files

File	Change
Modifiers/MarkdownLinkRendererModifier.swift	new — public View.markdownLinkRenderer(_:forURLScheme:)
Renderers/Node Representations/Links/Protocol/MarkdownLinkRenderer.swift	new — protocol + Configuration + AnyMarkdownLinkRenderer
Configurations/MarkdownRendererConfiguration.swift	adds linkRenderers field, contributes to Equatable
Renderers/Cmark/CmarkNodeVisitor.swift	visitLink dispatches to custom renderer if registered for the URL scheme (or "*"); default branch adds .help(url) for the native macOS hover tooltip

170 insertions, 1 deletion. No changes to existing public API surface.

Verification

• swift build — Build complete!
• Used in production by an external macOS app (@MainActor SwiftUI consumer) — favicon + URL hover tooltip on inline markdown links in an AI chat surface. Renders thousands of links per session, no observed cache thrash or render glitches.

MarkdownView 3 / #132

I see #132 ("Introducing MarkdownView 3") is in draft. If v3 plans to ship a link-renderer hook with a different architecture, happy to defer this — just let me know. Otherwise this should be useful for everyone on the current 2.x line.

[ajram23]

@LiYanan2004 Codex app does this and make for better looking links.