Add minimal string constant support to the WASM backend:
- Allow `ConstValue::String` in codegen by embedding UTF-8 string bytes and constructing a `StringBox` with `[data_ptr,length]`.
- Provide a minimal debugging import `env.print_str(ptr,len)` to verify strings at runtime.
- Unblock Issue #62 implementation and tests that require strings.
## Scope
Minimal features required:
1) Data segments for string literals
- Extend `WasmModule` (in `codegen.rs`) with a `data_segments: Vec<String>` field.
- Update `to_wat()` to emit `(data ...)` after memory/globals and before functions/exports.
- For each string constant, create a unique offset and emit a `(data (i32.const <offset>) "...bytes...")` entry.
2) Codegen for `ConstValue::String`
- In `generate_const`, when encountering `ConstValue::String(s)`,
- Allocate a data segment for `s` (UTF-8 bytes) and get its offset and length.
- Allocate a `StringBox` using existing helpers (see `MemoryManager`),
then set its fields: `data_ptr` and `length`.
- Return the `StringBox` pointer (i32) in the destination local.
3) Helper for `StringBox` allocation
- Either:
- Provide a dedicated WAT helper function `$alloc_stringbox` that calls `$malloc`, writes header (`type_id=0x1001`, `ref_count=1`, `field_count=2`), and returns the box pointer, then inline store `data_ptr`/`length`.
- Or:
- Use `$box_alloc` with `(type_id=0x1001, field_count=2)` and then store `data_ptr`/`length` via generated `i32.store` sequences.
4) Runtime import for string output (for verification)
- In host (Node/Browser), implement `importObject.env.print_str = (ptr,len) => { decode UTF-8 from memory; console.log(...) }`.
5) E2E test
- Add a tiny program that produces/prints a string (e.g., Const String → call `env.print_str(ptr,len)` via a minimal MIR program) and verify it logs the correct text.
- Option: update `test_runner.js` to include `print_str` and decode from memory using `TextDecoder('utf-8')`.
