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/todo-app/main.lux

@@ -0,0 +1,123 @@
+// Todo App - Stress tests ADTs, pattern matching, and list operations
+//
+// This demonstrates:
+// - ADTs for data modeling
+// - Pattern matching
+// - List operations
+// - Recursive display
+
+// Todo item
+type Priority =
+    | Low
+    | Medium
+    | High
+
+type TodoItem =
+    | TodoItem(Int, String, Bool, Priority)
+
+// Helper functions for TodoItem
+fn getId(item: TodoItem): Int =
+    match item { TodoItem(id, _, _, _) => id }
+
+fn getDescription(item: TodoItem): String =
+    match item { TodoItem(_, desc, _, _) => desc }
+
+fn isCompleted(item: TodoItem): Bool =
+    match item { TodoItem(_, _, completed, _) => completed }
+
+fn getPriority(item: TodoItem): Priority =
+    match item { TodoItem(_, _, _, priority) => priority }
+
+fn setCompleted(item: TodoItem, completed: Bool): TodoItem =
+    match item { TodoItem(id, desc, _, priority) => TodoItem(id, desc, completed, priority) }
+
+// Priority helpers
+fn priorityToString(p: Priority): String =
+    match p {
+        Low => "Low",
+        Medium => "Medium",
+        High => "High"
+    }
+
+fn priorityToSymbol(p: Priority): String =
+    match p {
+        Low => ".",
+        Medium => "*",
+        High => "!"
+    }
+
+// Format a todo item for display
+fn formatItem(item: TodoItem): String =
+    toString(getId(item)) + ". " + (if isCompleted(item) then "[x]" else "[ ]") + " " + priorityToSymbol(getPriority(item)) + " " + getDescription(item)
+
+// Display functions using recursion
+fn printItemsHelper(items: List<TodoItem>): Unit with {Console} =
+    match List.head(items) {
+        None => (),
+        Some(item) => {
+            Console.print("  " + formatItem(item))
+            match List.tail(items) {
+                None => (),
+                Some(rest) => printItemsHelper(rest)
+            }
+        }
+    }
+
+fn displayItems(items: List<TodoItem>): Unit with {Console} =
+    if List.isEmpty(items) then Console.print("  (no items)")
+    else printItemsHelper(items)
+
+fn countCompleted(items: List<TodoItem>): Int =
+    List.length(List.filter(items, isCompleted))
+
+fn displayStats(items: List<TodoItem>): Unit with {Console} =
+    Console.print("Total: " + toString(List.length(items)) + " | Completed: " + toString(countCompleted(items)) + " | Pending: " + toString(List.length(items) - countCompleted(items)))
+
+// Sample data - all on one line to avoid multiline list parse issues
+fn createSampleData(): List<TodoItem> = [TodoItem(1, "Learn Lux basics", true, High), TodoItem(2, "Build a project", false, High), TodoItem(3, "Read documentation", true, Medium), TodoItem(4, "Write tests", false, Medium), TodoItem(5, "Share with team", false, Low)]
+
+// Filter functions
+fn filterCompleted(items: List<TodoItem>): List<TodoItem> =
+    List.filter(items, isCompleted)
+
+fn filterPending(items: List<TodoItem>): List<TodoItem> =
+    List.filter(items, fn(item: TodoItem): Bool => isCompleted(item) == false)
+
+// Main demo
+fn main(): Unit with {Console} = {
+    Console.print("========================================")
+    Console.print("         TODO APP DEMO")
+    Console.print("========================================")
+    Console.print("")
+
+    let items = createSampleData()
+
+    Console.print("All Todo Items:")
+    Console.print("---------------")
+    displayItems(items)
+    Console.print("")
+    displayStats(items)
+    Console.print("")
+
+    Console.print("Completed Items:")
+    Console.print("----------------")
+    displayItems(filterCompleted(items))
+    Console.print("")
+
+    Console.print("Pending Items:")
+    Console.print("--------------")
+    displayItems(filterPending(items))
+    Console.print("")
+
+    Console.print("Marking item 2 as complete...")
+    let updatedItems = List.map(items, fn(item: TodoItem): TodoItem => if getId(item) == 2 then setCompleted(item, true) else item)
+    Console.print("")
+
+    Console.print("Updated Todo List:")
+    Console.print("------------------")
+    displayItems(updatedItems)
+    Console.print("")
+    displayStats(updatedItems)
+}
+
+let output = run main() with {}