Get rid of lowdown!

`cmark` is a more robust library for constructing Markdown than
`lowdown`. It in particular has the functionality we need to compute the
doc we need to compute. It is also more portable.

For terminal output, there is a new A.I. slop library has the same
license as the C code from lowdown upon which it is based. This keeps
our options open for upstreaming, if we want to do that.

Also, the `builtins.fetchTree` docs are now assembled using the cmark
AST rather than janky string indenting.

src/libcmarkcpp/include/cmark/cmark-terminal.hh

@@ -0,0 +1,108 @@
+#pragma once
+///@file
+
+#include "cmark-cpp.hh"
+
+#include <string>
+#include <vector>
+#include <memory>
+
+namespace cmark {
+
+/**
+ * Terminal rendering options
+ */
+struct TerminalOptions
+{
+    /** Terminal width in columns */
+    size_t cols = 80;
+
+    /** Content width (0 = auto, max 80 or cols) */
+    size_t width = 0;
+
+    /** Horizontal margin (left padding) */
+    size_t hmargin = 0;
+
+    /** Horizontal padding (additional left padding) */
+    size_t hpadding = 4;
+
+    /** Vertical margin (blank lines before/after) */
+    size_t vmargin = 0;
+
+    /** Center content */
+    bool centre = false;
+
+    /** Disable ANSI escape sequences */
+    bool noAnsi = false;
+
+    /** Disable ANSI colors only */
+    bool noColor = false;
+
+    /** Don't show any link URLs */
+    bool noLink = false;
+
+    /** Don't show relative link URLs */
+    bool noRelLink = false;
+
+    /** Shorten long URLs */
+    bool shortLink = false;
+};
+
+/**
+ * Style attributes for terminal output
+ */
+struct Style
+{
+    bool italic = false;
+    bool strike = false;
+    bool bold = false;
+    bool under = false;
+    size_t bcolour = 0; // Background color (ANSI code)
+    size_t colour = 0;  // Foreground color (ANSI code)
+    int override = 0;   // Override flags
+
+    static constexpr int OVERRIDE_UNDER = 0x01;
+    static constexpr int OVERRIDE_BOLD = 0x02;
+
+    bool hasStyle() const
+    {
+        return colour || bold || italic || under || strike || bcolour || override;
+    }
+};
+
+/**
+ * Terminal renderer for CommonMark documents
+ *
+ * Renders a CMark AST to ANSI terminal output with styling, wrapping,
+ * and proper indentation.
+ */
+class TerminalRenderer
+{
+public:
+    /**
+     * Create a new terminal renderer with the given options
+     */
+    explicit TerminalRenderer(const TerminalOptions & opts = TerminalOptions{});
+
+    ~TerminalRenderer();
+
+    // Non-copyable
+    TerminalRenderer(const TerminalRenderer &) = delete;
+    TerminalRenderer & operator=(const TerminalRenderer &) = delete;
+
+    /**
+     * Render a CMark node tree to a string
+     */
+    std::string render(cmark::Node & root);
+
+private:
+    struct Impl;
+    std::unique_ptr<Impl> impl;
+};
+
+/**
+ * Convenience function to render a CMark document to terminal output
+ */
+std::string renderTerminal(cmark::Node & root, const TerminalOptions & opts = TerminalOptions{});
+
+} // namespace cmark