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/backend/aot/compiler.rs

@@ -53,6 +53,8 @@ impl AotCompiler {
                 WasmError::UnsupportedInstruction(msg) => AotError::CompilationError(format!("Unsupported MIR instruction: {}", msg)),
                 WasmError::WasmValidationError(msg) => AotError::CompilationError(format!("WASM validation failed: {}", msg)),
                 WasmError::IOError(msg) => AotError::IOError(msg),
+                WasmError::RuntimeError(msg) => AotError::RuntimeError(msg),
+                WasmError::CompilationError(msg) => AotError::CompilationError(msg),
             })?;
         
         self.stats.wasm_size = wasm_bytes.len();