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>

local_tests/test_wasm_execution.sh

@@ -0,0 +1,24 @@
+#!/bin/bash
+# WASM execution test script
+
+echo "🎯 Testing WASM compilation and execution with host functions"
+
+# First compile to WAT
+echo "📝 Compiling test_mir_simple.nyash to WASM..."
+../target/release/nyash --compile-wasm test_mir_simple.nyash
+
+# Check if WAT was generated
+if [ -f "test_mir_simple.wat" ]; then
+    echo "✅ WAT file generated successfully"
+    echo "📄 WAT content preview:"
+    head -20 test_mir_simple.wat
+    echo "..."
+    
+    # Now we need a custom WASM runner that provides host functions
+    echo ""
+    echo "🚀 To execute WASM with host functions, we need to build a custom runner"
+    echo "   that provides the required imports (env::print, etc.)"
+else
+    echo "❌ WAT file generation failed"
+    exit 1
+fi