Tiny Language Compiler

# Tiny Language Compiler PRD

## Overview
Build a compiler for a small, custom programming language. The language should support variables, functions, control flow, and basic expressions.

## Language Spec
```
let x = 42;
let name = "world";

fn greet(who) {
  print("Hello, " + who);
}

fn factorial(n) {
  if (n <= 1) { return 1; }
  return n * factorial(n - 1);
}

greet(name);
print(factorial(5));

let i = 0;
while (i < 10) {
  print(i);
  i = i + 1;
}
```

## Requirements
- **Lexer**: Tokenize source into tokens (keywords, identifiers, numbers, strings, operators, punctuation)
- **Parser**: Build an AST from tokens (recursive descent or Pratt parser)
- **AST nodes**: LetStatement, FnDeclaration, IfExpression, WhileLoop, ReturnStatement, BinaryExpr, CallExpr, Assignment
- **Code generator**: Emit JavaScript source OR a custom bytecode with a VM
- **Types**: numbers (floats), strings, booleans, null
- **Operators**: +, -, *, /, %, ==, !=, <, >, <=, >=, &&, ||, !
- **Built-ins**: print(), len(), type()
- **Error reporting**: Line and column numbers in error messages
- **CLI**: compile run <file> executes the program, compile emit <file> prints generated code

## Tech Stack
- Any language — no parser generator libraries (no ANTLR, PEG.js, etc.)

## Scoring Criteria
- **Functional (40%)**: Lexer, parser, and codegen work end-to-end, programs execute correctly
- **Quality (20%)**: Clean compiler architecture, good error messages, handles edge cases
- **Fidelity (25%)**: All language features implemented, CLI works
- **Speed (15%)**: Time bonus