Add macOS support with dedicated NSTextView-based paragraph rendering

[Jun Yan (junyan72)]

Summary

Add macOS 14+ support to SwiftStreamingMarkdown by creating a dedicated AppKit rendering layer for paragraph content (the only component that required UIKit interop), while adapting all shared source files with platform typealiases and #if os guards.

Key pieces:

• ParagraphNSView — NSTextView subclass with CADisplayLink-based fade animation, context menu, and citation/latex attachment view providers
• ParagraphViewMac — NSViewRepresentable wrapper (macOS equivalent of ParagraphView)
• PlatformTypes.swift — Central typealias file (PlatformFont, PlatformColor, PlatformImage, PlatformEdgeInsets) enabling shared code to compile on both platforms
• Cross-platform InlineCitationAttachment — Rewritten with InterfaceStyle enum and platform-conditional image rendering
• Sample app — Updated with supportedDestinations: [iOS, macOS] and platform-conditional UI (NSAlert, NSSharingServicePicker, NSPasteboard)

Known limitations (follow-up work):

• BlockMathView on macOS is a placeholder Text() (iosMath supports macOS but wiring is deferred)
• Snapshot tests not yet recorded for macOS
• NSFont Sendable warnings exist (pre-existing pattern, not a regression)

Refs #85

Validation

• swift build — macOS library build succeeds
• xcodebuild build -destination 'platform=macOS' — sample app builds for macOS
• xcodebuild build -destination 'platform=iOS Simulator,name=iPhone 16' — sample app builds for iOS (no regression)
• make lint and make test not yet run on this branch (draft PR)

OSS readiness

• No secrets, internal URLs, private identifiers, or product-only service names were added.
• Public docs, fixtures, or notices were updated if behavior or dependencies changed.
• Third-party dependency changes (adds, removes, version bumps) are intentional and reviewed. (no dependency changes)
• Streaming/incomplete markdown behavior remains covered by fixtures or tests. (macOS snapshot tests pending)

[Jun Yan (junyan72)]

Closing — all macOS support functionality from this prototype has been incorporated into main via PR #106 and related PRs. The implementation uses a different file organization (separate MDFont/MDColor/MDImage types instead of PlatformTypes, UI/Paragraph/AppKit/ instead of UI/AppKit/) but covers the same functionality.