fix: move top-level let initialization into main() in C backend

Top-level let bindings with function calls (e.g., `let result = factorial(10)`)
were emitted as static initializers, which is invalid C since function calls
aren't compile-time constants. Now globals are declared with zero-init and
initialized inside main() before any run expressions execute.

Also fixes validate.sh to use exit codes instead of grep for cargo check/build.

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

src/codegen/c_backend.rs

@@ -279,7 +279,7 @@ impl CBackend {
                 Declaration::Let(let_decl) => {
                     // Skip run expressions - they're handled in the main wrapper
                     if !matches!(&let_decl.value, Expr::Run { .. }) {
-                        self.emit_global_let(&let_decl.name, &let_decl.value)?;
+                        self.emit_global_let(&let_decl.name)?;
                     }
                 }
                 _ => {}
@@ -5561,9 +5561,9 @@ impl CBackend {
         }
     }
 
-    fn emit_global_let(&mut self, name: &Ident, value: &Expr) -> Result<(), CGenError> {
-        let val = self.emit_expr(value)?;
-        self.writeln(&format!("static LuxInt {} = {};", name.name, val));
+    fn emit_global_let(&mut self, name: &Ident) -> Result<(), CGenError> {
+        // Declare global variable without initializer (initialized in main)
+        self.writeln(&format!("static LuxInt {} = 0;", name.name));
         self.writeln("");
         Ok(())
     }
@@ -5574,12 +5574,16 @@ impl CBackend {
             matches!(d, Declaration::Function(f) if f.name.name == "main")
         });
 
-        // Check for top-level run expressions
+        // Check for top-level run expressions or let bindings
         let has_run = program.declarations.iter().any(|d| {
             matches!(d, Declaration::Let(let_decl) if matches!(&let_decl.value, Expr::Run { .. }))
         });
 
-        if has_main || has_run {
+        let has_global_lets = program.declarations.iter().any(|d| {
+            matches!(d, Declaration::Let(let_decl) if !matches!(&let_decl.value, Expr::Run { .. }))
+        });
+
+        if has_main || has_run || has_global_lets {
             self.writeln("int main(int argc, char** argv) {");
             self.indent += 1;
 
@@ -5588,6 +5592,16 @@ impl CBackend {
             self.writeln("lux_argv = argv;");
             self.writeln("");
 
+            // Initialize top-level let bindings (non-run) inside main
+            for decl in &program.declarations {
+                if let Declaration::Let(let_decl) = decl {
+                    if !matches!(&let_decl.value, Expr::Run { .. }) {
+                        let val = self.emit_expr(&let_decl.value)?;
+                        self.writeln(&format!("{} = {};", let_decl.name.name, val));
+                    }
+                }
+            }
+
             // Execute top-level let bindings with run expressions
             // Track if main was already called via a run expression
             let mut main_called_via_run = false;