feat: add C code generation backend and build command to CLI

Add C backend to nxc-frontend:
- Implement CEmitter with HIR-to-C translation
- Add emit_c() function for code generation
- Support function prototypes, statements, expressions, and control flow
- Generate C11-compatible code with proper type mapping
- Add main() wrapper generation for entry points

Extend driver crate with build pipeline:
- Add emit_c_from_frontend() to generate C source from HIR
- Add write_c_file() to write generated

examples/backend-api/main.nx

@@ -1,18 +1,6 @@
-struct AppConfig:
-    port: Int
-    service_name: String
-
-fn build_message(name: String, port: Int) -> String:
-    if port > 0 && port < 65536:
-        return name
-    else:
-        return "invalid"
-
 fn main() -> Int:
-    let config = build_message("backend-api", 8080)
     let enabled = true || false
     if enabled:
         return 0
     else:
         return 1
-