feat: implement md-to-html-swift converter

[PsychQuantClaw]

Summary

Implement a new Layer 3 Swift package: md-to-html-swift for direct Markdown → HTML conversion.

This is the reverse pair of the in-flight html-to-md-swift work and is already promoted to P0 by the conversion matrix rules.

Why

• html-to-md-swift makes the reverse path architecturally straightforward.
• Direct Markdown → HTML avoids routing through other hub formats.
• The CLI should expose a first-class Markdown → HTML command instead of forcing downstream tools to shell out to unrelated renderers.

Proposed package shape

• New package: packages/md-to-html-swift
• Layer: Layer 3 (converter/renderer)
• Product: MDToHTMLSwift
• Platform: Swift 5.9+, macOS 14+

Dependencies

Layer 2

• doc-converter-swift
  • use DocumentConverter
  • use StreamingOutput
  • use shared ConversionOptions / ConversionError

Layer 1 / parsing

• swift-markdown (Apple) for Markdown AST parsing
  • gives a structured parser instead of regex-based rendering
  • keeps the converter target-aware and easier to test

If swift-markdown proves too heavy for the initial cut, start with a minimal internal parser for the supported subset and keep the package boundary compatible with a future parser swap.

Implementation sketch

• Add MarkdownToHTMLConverter that conforms to DocumentConverter
• Parse Markdown input into an AST / block sequence
• Emit HTML through StreamingOutput incrementally
• Preserve streaming-friendly output APIs (convert(input:output:), convertToString(...))
• Support core Markdown constructs first:
  • headings
  • paragraphs
  • emphasis / strong / code spans
  • fenced code blocks
  • blockquotes
  • ordered / unordered lists
  • links / images
  • horizontal rules

CLI integration

Add a new macdoc md to-html command (or equivalent Markdown command group consistent with existing CLI conventions).

Test strategy

• Unit tests for block-level rendering
• Unit tests for inline rendering and escaping
• Fixture tests for nested lists, blockquotes, code fences, links, and images
• CLI smoke test that verifies output file creation and representative HTML fragments
• Aim for 80%+ coverage in the new package

Acceptance criteria

• swift test passes from repo root
• new package builds independently
• CLI exposes Markdown → HTML conversion
• CONVERSIONS.md is updated from planned → active/implemented as work progresses