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/interpreter.rs

@@ -176,6 +176,11 @@ pub enum Value {
     },
     /// JSON value (for JSON parsing/manipulation)
     Json(serde_json::Value),
+    /// Extern function (FFI — only callable from JS backend)
+    ExternFn {
+        name: String,
+        arity: usize,
+    },
 }
 
 impl Value {
@@ -197,6 +202,7 @@ impl Value {
             Value::Constructor { .. } => "Constructor",
             Value::Versioned { .. } => "Versioned",
             Value::Json(_) => "Json",
+            Value::ExternFn { .. } => "ExternFn",
         }
     }
 
@@ -407,6 +413,7 @@ impl fmt::Display for Value {
                 write!(f, "{} @v{}", value, version)
             }
             Value::Json(json) => write!(f, "{}", json),
+            Value::ExternFn { name, .. } => write!(f, "<extern fn {}>", name),
         }
     }
 }
@@ -1405,6 +1412,25 @@ impl Interpreter {
                 Ok(Value::Unit)
             }
 
+            Declaration::ExternFn(ext) => {
+                // Register a placeholder that errors at runtime
+                let name = ext.name.name.clone();
+                let arity = ext.params.len();
+                // Create a closure that produces a clear error
+                let closure = Closure {
+                    params: ext.params.iter().map(|p| p.name.name.clone()).collect(),
+                    body: Expr::Literal(crate::ast::Literal {
+                        kind: crate::ast::LiteralKind::Unit,
+                        span: ext.span,
+                    }),
+                    env: self.global_env.clone(),
+                };
+                // We store an ExternFn marker value
+                self.global_env
+                    .define(&name, Value::ExternFn { name: name.clone(), arity });
+                Ok(Value::Unit)
+            }
+
             Declaration::Effect(_) | Declaration::Trait(_) | Declaration::Impl(_) => {
                 // These are compile-time only
                 Ok(Value::Unit)
@@ -1924,6 +1950,13 @@ impl Interpreter {
                 }))
             }
             Value::Builtin(builtin) => self.eval_builtin(builtin, args, span),
+            Value::ExternFn { name, .. } => Err(RuntimeError {
+                message: format!(
+                    "Extern function '{}' can only be called when compiled to JavaScript (use `lux build --target js`)",
+                    name
+                ),
+                span: Some(span),
+            }),
             v => Err(RuntimeError {
                 message: format!("Cannot call {}", v.type_name()),
                 span: Some(span),