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/wasm/mod.rs

@@ -8,10 +8,13 @@
 mod codegen;
 mod memory;
 mod runtime;
+mod host;
+mod executor;
 
 pub use codegen::{WasmCodegen, WasmModule};
 pub use memory::{MemoryManager, BoxLayout};
 pub use runtime::RuntimeImports;
+pub use executor::WasmExecutor;
 
 use crate::mir::MirModule;
 
@@ -23,6 +26,8 @@ pub enum WasmError {
     UnsupportedInstruction(String),
     WasmValidationError(String),
     IOError(String),
+    RuntimeError(String),
+    CompilationError(String),
 }
 
 impl std::fmt::Display for WasmError {
@@ -33,6 +38,8 @@ impl std::fmt::Display for WasmError {
             WasmError::UnsupportedInstruction(msg) => write!(f, "Unsupported instruction: {}", msg),
             WasmError::WasmValidationError(msg) => write!(f, "WASM validation error: {}", msg),
             WasmError::IOError(msg) => write!(f, "IO error: {}", msg),
+            WasmError::RuntimeError(msg) => write!(f, "Runtime error: {}", msg),
+            WasmError::CompilationError(msg) => write!(f, "Compilation error: {}", msg),
         }
     }
 }