Extension check logic edge case

❌ Problem
```
filepath.size() >= 5 &&
filepath.substr(filepath.size() - 5) == ".wasm"
```


Fails for:

- > uppercase .WASM
- > paths like file.wasm.bak

✅ Fix (robust & cleaner)
```
fs::path p(filepath);
return p.extension() == ".wasm";
```


📌 Severity: Minor
📌 Phase impact: Phase 6 robustness