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/types.rs

@@ -964,6 +964,11 @@ impl TypeEnv {
                         ],
                         return_type: Type::Unit,
                     },
+                    EffectOpDef {
+                        name: "glob".to_string(),
+                        params: vec![("pattern".to_string(), Type::String)],
+                        return_type: Type::List(Box::new(Type::String)),
+                    },
                 ],
             },
         );