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/typechecker.rs

@@ -5,9 +5,9 @@
 use std::collections::HashMap;
 
 use crate::ast::{
-    self, BinaryOp, Declaration, EffectDecl, Expr, FunctionDecl, HandlerDecl, Ident, ImplDecl,
-    ImportDecl, LetDecl, Literal, LiteralKind, MatchArm, Parameter, Pattern, Program, Span,
-    Statement, TraitDecl, TypeDecl, TypeExpr, UnaryOp, VariantFields,
+    self, BinaryOp, Declaration, EffectDecl, ExternFnDecl, Expr, FunctionDecl, HandlerDecl, Ident,
+    ImplDecl, ImportDecl, LetDecl, Literal, LiteralKind, MatchArm, Parameter, Pattern, Program,
+    Span, Statement, TraitDecl, TypeDecl, TypeExpr, UnaryOp, VariantFields,
 };
 use crate::diagnostics::{find_similar_names, format_did_you_mean, Diagnostic, ErrorCode, Severity};
 use crate::exhaustiveness::{check_exhaustiveness, missing_patterns_hint};
@@ -1227,6 +1227,17 @@ impl TypeChecker {
                 let trait_impl = self.collect_impl(impl_decl);
                 self.env.trait_impls.push(trait_impl);
             }
+            Declaration::ExternFn(ext) => {
+                // Register extern fn type signature (like a regular function but no body)
+                let param_types: Vec<Type> = ext
+                    .params
+                    .iter()
+                    .map(|p| self.resolve_type(&p.typ))
+                    .collect();
+                let return_type = self.resolve_type(&ext.return_type);
+                let fn_type = Type::function(param_types, return_type);
+                self.env.bind(&ext.name.name, TypeScheme::mono(fn_type));
+            }
         }
     }