fix: module syntax — changed src_bux/*.bux from module X; to module X { ... }

src_bux/c_backend.bux

@@ -1,6 +1,6 @@
 // c_backend.bux — C transpiler backend (ported from c_backend.nim)
 // Generates C code from the HIR.
-module CBackend;
+module CBackend {
 
 // ---------------------------------------------------------------------------
 // Type → C type name
@@ -262,4 +262,4 @@ func CBackend_Generate(mod: *HirModule) -> String {
 func CBackend_Free(cbe: *CEmitter) {
     StringBuilder_Free(&cbe.sb);
     bux_free(cbe as *void);
-}
+}