…ource_files (#1232)

This is a fix for: write_source_files: wrong runfiles_dir assignment
with runfiles enabled for windows #1223

The working dir differs when runfiles are enabled. On windows the
variables are not case sensitive, so that an assignment to
"runfiles_dir" overwrites the "RUNFILES_DIR" env var (which is wrong,
when runfiles are enabled).

Windows cmd has a known bug where GOTO/CALL to labels fails when batch
files use LF-only line endings and the label crosses a 512-byte boundary
during parsing. This causes "cannot find batch label" errors like, for
`diff_test`:
```
The system cannot find the batch label specified - compare_files
```

References:
- https://www.dostips.com/forum/viewtopic.php?t=8988
- rocq-prover/rocq#8610

This fix ensures all generated .bat files use CRLF line endings by
converting templates and using \r\n in string replacements throughout:
- diff_test: template and substitutions,
- write_source_file: batch updater scripts,
- windows_utils: native launcher scripts.

To verify these changes work correctly across platforms, we add a Go
binary (`check_newlines`) that validates line endings:
- on Windows: verifies scripts have proper CRLF line endings,
- otherwise: verifies \n to \r\n replacements don't affect POSIX
scripts.

Note: we initially considered `sh_test` with PowerShell/batch wrappers,
but encountered issues with interpreter dependencies, script
portability, and symlink handling across platforms.
A single Go binary invoked via `native_test` proved more reliable and
maintainable, working consistently across all platforms (no new external
dependencies either).