feat: add stress test projects and testing documentation

- Add String.fromChar function to convert Char to String
- Create four stress test projects demonstrating Lux features:
  - json-parser: recursive descent parsing with Char handling
  - markdown-converter: string manipulation and ADTs
  - todo-app: list operations and pattern matching
  - mini-interpreter: AST evaluation and environments
- Add comprehensive testing documentation (docs/testing.md)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

projects/markdown-converter/main.lux

@@ -0,0 +1,79 @@
+// Markdown to HTML Converter - Stress tests string manipulation and parsing
+//
+// This demonstrates:
+// - String parsing and manipulation
+// - ADTs for document structure
+// - Recursive processing
+// - Pattern matching on strings
+
+// Markdown AST
+type MarkdownNode =
+    | MHeading(Int, String)
+    | MParagraph(String)
+    | MList(Bool, List<String>)
+    | MHorizontalRule
+
+// Parse a line to determine block type
+fn parseBlockType(line: String): MarkdownNode =
+    if String.startsWith(line, "# ") then
+        MHeading(1, String.substring(line, 2, String.length(line)))
+    else if String.startsWith(line, "## ") then
+        MHeading(2, String.substring(line, 3, String.length(line)))
+    else if String.startsWith(line, "### ") then
+        MHeading(3, String.substring(line, 4, String.length(line)))
+    else if String.startsWith(line, "---") then
+        MHorizontalRule
+    else if String.startsWith(line, "- ") then
+        MList(false, [String.substring(line, 2, String.length(line))])
+    else
+        MParagraph(line)
+
+// Convert to HTML
+fn nodeToHtml(node: MarkdownNode): String =
+    match node {
+        MHeading(level, text) => "<h" + toString(level) + ">" + text + "</h" + toString(level) + ">",
+        MParagraph(text) => if text == "" then "" else "<p>" + text + "</p>",
+        MList(ordered, items) => {
+            let tag = if ordered then "ol" else "ul"
+            let itemsHtml = List.map(items, fn(item: String): String => "<li>" + item + "</li>")
+            "<" + tag + ">" + String.join(itemsHtml, "") + "</" + tag + ">"
+        },
+        MHorizontalRule => "<hr />"
+    }
+
+// Convert markdown string to HTML
+fn convert(markdown: String): String = {
+    let lines = String.lines(markdown)
+    let nodes = List.map(lines, parseBlockType)
+    let htmlLines = List.filter(List.map(nodes, nodeToHtml), fn(s: String): Bool => s != "")
+    String.join(htmlLines, "\n")
+}
+
+// Test the converter
+fn main(): Unit with {Console} = {
+    Console.print("========================================")
+    Console.print("     MARKDOWN TO HTML CONVERTER")
+    Console.print("========================================")
+    Console.print("")
+
+    Console.print("Test 1: Headings")
+    Console.print(convert("# Heading 1"))
+    Console.print(convert("## Heading 2"))
+    Console.print(convert("### Heading 3"))
+    Console.print("")
+
+    Console.print("Test 2: Paragraphs")
+    Console.print(convert("This is a paragraph."))
+    Console.print(convert("Another paragraph here."))
+    Console.print("")
+
+    Console.print("Test 3: Lists")
+    Console.print(convert("- Item 1"))
+    Console.print(convert("- Item 2"))
+    Console.print("")
+
+    Console.print("Test 4: Horizontal rule")
+    Console.print(convert("---"))
+}
+
+let output = run main() with {}