Load imperfect images and allow non-standard file names (#11, #12)

[TobyLobster]

Issue #11 - images that fail to open:

• An image whose size is not a multiple of 256 bytes (incomplete last sector) is now loaded with the last sector padded, instead of failing with "invalid disk image size".
• An image that is shorter than the catalog implies no longer fails with "disk image too small". Previously an invalid catalog (e.g. a non-standard sector count such as 293 or 320) made dfsimage demand a full-size image even when all file data was present. The check now uses the extents of the catalog entries; missing data reads as empty (0xE5) sectors, as in emulators, with a ValidationWarning.
• An image too short to hold the catalog is still an error.

Issue #12 - non-standard characters in file names:

• New Image.allow_invalid_names setting (class default, per-instance, or allow_invalid_names= on Image(), Image.open() and Image.create()), and a --allow-invalid-names CLI flag. When set, file and directory names containing 7-bit codes outside 32-126 (e.g. control codes used for copy protection) can be created, renamed and imported with one warning per name. Codes 128+ are still rejected. The default is unchanged.
• Inf file names keep leading/trailing whitespace-class characters such as form feed, which are part of such names.

Also fixes inf files being ignored on import unless the current working directory was the directory containing them.

Claude-Session: https://claude.ai/code/session_01QHzHRcSa5fqh4a17f2Uy2k