feat: add html-to-md-swift converter and CLI support

[PsychQuantClaw]

Summary

• add new Layer 3 package packages/html-to-md-swift
• implement HTMLConverter: DocumentConverter using SwiftSoup + streaming Markdown emission
• add macdoc html CLI subcommand
• populate CONVERSIONS.md with a real matrix + priority queue and mark html-to-md-swift active

Closes #13.

What the converter supports

Block-level

• headings h1...h6
• paragraphs
• unordered / ordered lists
• blockquote
• fenced code blocks from pre > code
• horizontal rule
• tables
• common wrapper/container blocks (div, section, article, main, etc.)

Inline

• bold / emphasis / strikethrough
• inline code
• links
• images
• <br> hard-break option
• optional raw HTML preservation for <u>, <sup>, <sub>, <mark>

Testing

Validated the new package with:

cd packages/html-to-md-swift
swift build --target HTMLToMDSwift
swift run HTMLToMDSwiftSelfTest

Self-test coverage includes:

• headings + paragraphs
• inline formatting + links
• nested lists
• code fences + language detection
• blockquotes
• tables
• images + horizontal rule
• hard breaks
• optional HTML extensions
• frontmatter

Notes

• Root-level swift build is still affected by the existing clean-clone path dependency problem tracked in build: move local path dependencies to remote packages for clean clone/build #12; this PR does not attempt to solve that separately.
• Because the current toolchain on the dev machine lacks XCTest / Testing modules, the package uses a self-test executable for verification instead of swift test in this branch.

[kiki830621]

Code Review

CRITICAL

• packages/html-to-md-swift/Package.swift:23-28 — Self-test 宣告為 .executableTarget，不是 .testTarget。swift test 完全跑不到這些測試。專案規定 80% test coverage（common-testing.md），其他所有 package 都用 .testTarget + XCTestCase。需要改成標準 test target。

HIGH

• HTMLConverter.swift:17 — SwiftSoup.parse(html, input.deletingLastPathComponent().absoluteString) 把 file:///... 當 base URL 傳入，導致 HTML 中的相對連結（如 href="about.html"）被解析成 file:///Users/.../about.html 洩漏到 Markdown 輸出。應傳空字串 "" 以保留原始相對路徑。

• HTMLConverter.swift — 只支援 UTF-8 encoding（String(contentsOf: input, encoding: .utf8)）。實務上 HTML 常用 ISO-8859-1 / Windows-1252，會直接 throw I/O error。建議先嘗試 UTF-8，失敗後 fallback String(contentsOf: input, usedEncoding:)。

• HTMLConverter.swift:105-107 — <hr> 輸出 ---，與 --frontmatter 模式的 YAML 分隔符衝突。Markdown parser 會誤判。建議改用 * * * 或 - - -（帶空格）。

• Sources/MacDocCLI/MacDoc+HTML.swift:8 — 用 ParsableCommand 而非 AsyncParsableCommand，與 Word subcommand 模式不一致。

• packages/html-to-md-swift/Package.swift:5 — deployment target 宣告 .macOS(.v14)，其他所有 local packages 都是 .v13。應統一為 .v13。

MEDIUM

• <ol start="N"> 的 start attribute 被忽略，永遠從 1 開始編號。
• Block-level <br> 不受 --hard-breaks flag 控制，與 inline path 行為不一致。
• trimTrailingNewlines 用 removeLast() in-place mutation，可改用 trimmingCharacters(in:)。

[kiki830621]

Fixes applied and merged directly to main. See commit 60aec53.