feat: Apply Copilot's WASM UTF-8 fix manually

- Change from compile_module() to compile_to_wat() to fix UTF-8 error
- WASM compilation now outputs WAT text format directly
- Successfully generates WAT file for simple test case
- Note: Debug build still has issues (separate bug)

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

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

src/runner.rs

@@ -300,10 +300,10 @@ impl NyashRunner {
             }
         };
 
-        // Compile to WASM
+        // Compile to WASM (Phase 9.77a fix: use compile_to_wat instead of compile_module)
         let mut wasm_backend = WasmBackend::new();
-        let wasm_code = match wasm_backend.compile_module(compile_result.module) {
-            Ok(wasm) => wasm,
+        let wat_text = match wasm_backend.compile_to_wat(compile_result.module) {
+            Ok(wat) => wat,
             Err(e) => {
                 eprintln!("❌ WASM compilation error: {}", e);
                 process::exit(1);
@@ -321,14 +321,8 @@ impl NyashRunner {
             });
         let output_file = format!("{}.wat", output);
 
-        // Write WASM output
-        let output_str = match std::str::from_utf8(&wasm_code) {
-            Ok(s) => s,
-            Err(_) => {
-                eprintln!("❌ Generated WASM is not valid UTF-8");
-                process::exit(1);
-            }
-        };
+        // Write WAT output (already a string)
+        let output_str = wat_text;
         
         match fs::write(&output_file, output_str) {
             Ok(()) => {