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/core/basic.nx

@@ -0,0 +1,9 @@
+fn add(a: Int, b: Int) -> Int:
+    return a + b
+
+fn main() -> Int:
+    let value = add(20, 22)
+    if value == 42:
+        return 0
+    else:
+        return 1