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>

src/interpreter.rs

@@ -83,6 +83,7 @@ pub enum BuiltinFn {
     StringToUpper,
     StringToLower,
     StringSubstring,
+    StringFromChar,
 
     // JSON operations
     JsonParse,
@@ -809,6 +810,10 @@ impl Interpreter {
                 "substring".to_string(),
                 Value::Builtin(BuiltinFn::StringSubstring),
             ),
+            (
+                "fromChar".to_string(),
+                Value::Builtin(BuiltinFn::StringFromChar),
+            ),
         ]));
         env.define("String", string_module);
 
@@ -2293,6 +2298,17 @@ impl Interpreter {
                 Ok(EvalResult::Value(Value::String(result)))
             }
 
+            BuiltinFn::StringFromChar => {
+                if args.len() != 1 {
+                    return Err(err("String.fromChar requires 1 argument: char"));
+                }
+                let c = match &args[0] {
+                    Value::Char(c) => *c,
+                    v => return Err(err(&format!("String.fromChar expects Char, got {}", v.type_name()))),
+                };
+                Ok(EvalResult::Value(Value::String(c.to_string())))
+            }
+
             // JSON operations
             BuiltinFn::JsonParse => {
                 let s = Self::expect_arg_1::<String>(&args, "Json.parse", span)?;