Summary

• Add gerbv_is_loadable_file() content probe to detect gerber/drill/PnP files without relying on extensions
• Add scan_directory() to main.c that filters directory contents through the probe
• Modify the CLI file-loading loop to handle directory arguments alongside regular files

Closes gerbv#308

How it works

When a CLI argument is a directory, scan_directory() iterates its entries, probes each regular file with gerbv_is_loadable_file(), and collects matches into a sorted list. The probe reuses the exact same detection functions as gerbv_open_image (RS-274X, Excellon, PnP, RS-274D) — no extension guessing. Dot-prefixed files like .GBL and .GTL (Altium output) are included since detection is content-based.

Test plan

• cmake --build build — clean build, zero warnings
• ctest — 94/104 pass (unchanged baseline)
• gerbv test/inputs/ — loads all test gerber/drill files
• gerbv . from empty dir — prints "No loadable files found"
• gerbv file.gbr — single file still works
• gerbv --export=png -o out.png test/inputs/ — directory works with export
• Extensionless gerber in a directory is detected and loaded