feat: open directory of gerber files (gerbv .) #308
Description
Problem
Running gerbv . or gerbv ./gerbers/ treats the argument as a filename, which fails. There's no way to open all gerber files in a directory without listing them individually.
This is a familiar pattern from other tools — code . opens VS Code with the current directory's contents, vim . opens netrw, etc. gerbv should support the same convention: pass a directory and it loads everything it recognizes from it.
Why content-based detection matters
Gerber files have wildly inconsistent naming across EDA tools. Altium outputs files like .GBL, .GTL, .GTO (dot-prefixed, no basename). Some shops produce extensionless files. Extension-based filtering would miss these — the only reliable approach is to probe file contents using the same detection logic gerbv already uses internally (RS-274X, Excellon, pick-and-place, RS-274D signatures).
Expected behavior
| Command | Result |
|---|---|
| `gerbv .` | Load all recognized files from CWD |
| `gerbv ./gerbers/` | Load all recognized files from `gerbers/` |
| `gerbv file.gbr` | Unchanged — single file |
| `gerbv ./gerbers/ extra.gbr` | Directory contents + extra file |
| `gerbv --export=png -o out.png ./gerbers/` | Directory works with CLI export mode |
| `gerbv empty-dir/` | Prints "No loadable files found", opens empty |
- Non-matching files (READMEs, images, Makefiles) are silently skipped
- Dot-prefixed gerber files (`.GBL`, `.GTL`) are included
- Files are loaded in alphabetical order for predictable layer ordering
- Not recursive — only the specified directory level
Export mode
Directory arguments work with `--export` the same way individual files do. All files from the directory are loaded as layers and composited into a single output — the same stacked, color-coded view you see in the GUI:
```bash
These are equivalent:
gerbv --export=png -o board.png ./gerbers/
gerbv --export=png -o board.png gerbers/copper.gbr gerbers/mask.gbr gerbers/drill.drl
```
This is consistent with how `code .` works — it doesn't open each file in a separate window, it opens one workspace with all the files loaded. Similarly, `gerbv .` opens one viewer with all recognized layers loaded and stacked.