Fix Unreal C++ codegen module_prefix field missing from struct

The UnrealCpp struct was missing the module_prefix field that generate.rs
was trying to set, causing build failures. This adds the field and updates
all usages throughout unrealcpp.rs to properly use the module prefix.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: Ludv1gL

crates/cli/src/subcommands/generate.rs

@@ -25,7 +25,7 @@ use std::io::Read;
 pub fn cli() -> clap::Command {
     clap::Command::new("generate")
         .about("Generate client files for a spacetime module.")
-        .override_usage("spacetime generate --lang <LANG> --out-dir <DIR> [--project-path <DIR> | --bin-path <PATH> | --module-name <MODULE_NAME> | --uproject-dir <DIR> | --module-prefix <PREFIX>]")
+        .override_usage("spacetime generate --lang <LANG> --out-dir <DIR> [--project-path <DIR> | --bin-path <PATH> | --module-name <MODULE_NAME> | --uproject-dir <DIR>]")
         .arg(
             Arg::new("wasm_file")
                 .value_parser(clap::value_parser!(PathBuf))