fix(phase-4.3c-3): Fix StringBox literal handling in MIR builder

Phase 4-3c-3 Complete: WASM host functions now correctly output string content

## Changes:
- Fixed MIR builder to handle StringBox with string literal arguments
- Special case for  to generate proper string constants
- Removed debug output after successful verification
- WASM now correctly outputs "Hello MIR!" instead of "StringBox"

## Test Results:
- MIR generation: ✅ Generates  correctly
- WASM compilation: ✅ String data correctly placed at offset 4096
- WASM execution: ✅ Outputs "Hello MIR\!" as expected

## Technical Details:
- Modified build_new_expression() to detect StringBox with literal arguments
- Generates Const instruction with actual string content
- Host function reads StringBox memory layout correctly

This completes the WASM string output functionality for Phase 4.

🤖 Generated with Claude Code
Co-Authored-By: Claude <noreply@anthropic.com>

src/bin/nyash-wasm-run.rs

@@ -0,0 +1,51 @@
+/*!
+ * Nyash WASM Runner - Execute Nyash WASM modules with host functions
+ * 
+ * Phase 4-3c: Standalone WASM executor for testing
+ */
+
+use nyash_rust::backend::wasm::{WasmExecutor, WasmError};
+use std::env;
+use std::path::Path;
+
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    let args: Vec<String> = env::args().collect();
+    
+    if args.len() != 2 {
+        eprintln!("Usage: {} <file.wat>", args[0]);
+        eprintln!("");
+        eprintln!("Execute a Nyash WASM module with host functions");
+        std::process::exit(1);
+    }
+    
+    let wat_file = &args[1];
+    
+    if !Path::new(wat_file).exists() {
+        eprintln!("❌ File not found: {}", wat_file);
+        std::process::exit(1);
+    }
+    
+    println!("🚀 Nyash WASM Runner - Executing: {} 🚀", wat_file);
+    println!("");
+    
+    // Create executor
+    let executor = WasmExecutor::new()?;
+    
+    // Execute WAT file
+    match executor.execute_wat_file(wat_file) {
+        Ok(output) => {
+            if !output.is_empty() {
+                println!("📝 Program output:");
+                println!("{}", output);
+            }
+            println!("");
+            println!("✅ Execution completed successfully!");
+        },
+        Err(e) => {
+            eprintln!("❌ Execution error: {}", e);
+            std::process::exit(1);
+        }
+    }
+    
+    Ok(())
+}