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

crates/nxc-cli/Cargo.toml

@@ -18,3 +18,4 @@ path = "src/bin/nexacore.rs"
 
 [dependencies]
 nxc-driver = { path = "../nxc-driver" }
+nxc-frontend = { path = "../nxc-frontend" }