bug: wc output padding differs from real bash for all stdin inputs

[chaliy]

Summary

All 20+ tests in `wc.test.sh` are marked with `### bash_diff: Bashkit wc uses fixed-width padding for stdin, real bash uses no padding`. While functionally correct, scripts that parse `wc` output with exact string matching will fail.

Example

Real bash:
```
3 file.txt
```

Bashkit:
```
3 file.txt
```

Impact

Any script doing exact string comparison on `wc` output (e.g., `[ "$(wc -l < file)" = "3" ]`) may break due to leading whitespace. This is a common pattern in real-world scripts.

Notes

This might be intentional for consistency, but the large number of bash_diff markers suggests it's worth aligning with real bash behavior, at least for stdin inputs.

Acceptance criteria

• Evaluate whether to match real bash padding behavior
• If fixing: update wc output formatting for stdin
• Remove `### bash_diff` markers from wc.test.sh