feat: add File.glob for file pattern matching (issue 15)

Add File.glob(pattern) effect operation that returns a list of file
paths matching a glob pattern (e.g., "src/**/*.lux"). Implemented
across interpreter (using glob crate), JS backend (handler-based),
and C backend (using POSIX glob.h).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/interpreter.rs

@@ -3968,6 +3968,29 @@ impl Interpreter {
                 }
             }
 
+            ("File", "glob") => {
+                let pattern = match request.args.first() {
+                    Some(Value::String(s)) => s.clone(),
+                    _ => return Err(RuntimeError {
+                        message: "File.glob requires a string pattern".to_string(),
+                        span: None,
+                    }),
+                };
+                match glob::glob(&pattern) {
+                    Ok(paths) => {
+                        let entries: Vec<Value> = paths
+                            .filter_map(|entry| entry.ok())
+                            .map(|path| Value::String(path.to_string_lossy().to_string()))
+                            .collect();
+                        Ok(Value::List(entries))
+                    }
+                    Err(e) => Err(RuntimeError {
+                        message: format!("Invalid glob pattern '{}': {}", pattern, e),
+                        span: None,
+                    }),
+                }
+            }
+
             // ===== Process Effect =====
             ("Process", "exec") => {
                 use std::process::Command;