MASM source scan includes files under target directories

`collect_masm_files` recursively walks every directory under a target root and collects any file ending in `.masm`:

```rust
if path.is_dir() {
collect_masm_files(&path, files)?;
} else if path.extension().is_some_and(|ext| ext == "masm") {
files.push(normalize_path(&path));
}
```

For projects whose target source is at the package root, for example `path = "mod.masm"`, the target root is the project directory. In that layout, any generated `.masm` file under the conventional build output directory `target/` is indexed as source, which can pollute workspace symbols, definitions, and references.

The scanner should avoid descending into `target/` when collecting source `.masm` files.