feat: add extern fn declarations for JS FFI

Adds `extern fn` syntax for declaring external JavaScript functions:
  extern fn getElementById(id: String): Element
  extern fn getContext(el: Element, kind: String): CanvasCtx = "getContext"
  pub extern fn alert(msg: String): Unit

Changes across 11 files:
- Lexer: `extern` keyword
- AST: `ExternFnDecl` struct + `Declaration::ExternFn` variant
- Parser: parse `extern fn` with optional `= "jsName"` override
- Typechecker: register extern fn type signatures
- Interpreter: ExternFn value with clear error on call
- JS backend: emit extern fn calls using JS name (no _lux suffix)
- C backend: silently skips extern fns
- Formatter, linter, modules, symbol_table: handle new variant

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

src/codegen/js_backend.rs

@@ -71,6 +71,8 @@ pub struct JsBackend {
     var_substitutions: HashMap<String, String>,
     /// Effects actually used in the program (for tree-shaking runtime)
     used_effects: HashSet<String>,
+    /// Extern function names mapped to their JS names
+    extern_fns: HashMap<String, String>,
 }
 
 impl JsBackend {
@@ -93,6 +95,7 @@ impl JsBackend {
             has_handlers: false,
             var_substitutions: HashMap::new(),
             used_effects: HashSet::new(),
+            extern_fns: HashMap::new(),
         }
     }
 
@@ -112,6 +115,14 @@ impl JsBackend {
                 Declaration::Type(t) => {
                     self.collect_type(t)?;
                 }
+                Declaration::ExternFn(ext) => {
+                    let js_name = ext
+                        .js_name
+                        .clone()
+                        .unwrap_or_else(|| ext.name.name.clone());
+                    self.extern_fns.insert(ext.name.name.clone(), js_name);
+                    self.functions.insert(ext.name.name.clone());
+                }
                 _ => {}
             }
         }
@@ -2723,6 +2734,10 @@ impl JsBackend {
 
     /// Mangle a Lux name to a valid JavaScript name
     fn mangle_name(&self, name: &str) -> String {
+        // Extern functions use their JS name directly (no mangling)
+        if let Some(js_name) = self.extern_fns.get(name) {
+            return js_name.clone();
+        }
         format!("{}_lux", name)
     }