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_string_assignment.nyash

@@ -0,0 +1 @@
+local x = "Hello MIR\!"