feat: implement interactive debugger

Adds a REPL-based debugger with:
- Breakpoint management (set/delete by line number)
- Single-step and continue execution modes
- Source code listing with breakpoint markers
- Expression evaluation in debug context
- Variable inspection and call stack display

Usage: lux debug <file.lux>

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

src/main.rs

@@ -1,6 +1,7 @@
 //! Lux - A functional programming language with first-class effects
 
 mod ast;
+mod debugger;
 mod diagnostics;
 mod exhaustiveness;
 mod formatter;
@@ -121,6 +122,17 @@ fn main() {
                 }
                 check_file(&args[2]);
             }
+            "debug" => {
+                // Start debugger
+                if args.len() < 3 {
+                    eprintln!("Usage: lux debug <file.lux>");
+                    std::process::exit(1);
+                }
+                if let Err(e) = debugger::Debugger::run(&args[2]) {
+                    eprintln!("Debugger error: {}", e);
+                    std::process::exit(1);
+                }
+            }
             path => {
                 // Run a file
                 run_file(path);
@@ -142,6 +154,7 @@ fn print_help() {
     println!("  lux check <file.lux>  Type check without running");
     println!("  lux test [pattern]    Run tests (optional pattern filter)");
     println!("  lux watch <file.lux>  Watch and re-run on changes");
+    println!("  lux debug <file.lux>  Start interactive debugger");
     println!("  lux init [name]       Initialize a new project");
     println!("  lux --lsp             Start LSP server (for IDE integration)");
     println!("  lux --help            Show this help");