Allow explicit NTSC for VOB extraction

[Blackspirits]

Summary

VOB extraction currently assumes PAL timing and frame geometry. There is no reliable PAL/NTSC auto-detection from VOB data alone without IFO parsing, so NTSC DVDs need an explicit CLI choice.

This follow-up builds on the current multi-stream overwrite fix and:

• adds --vob-pal and --vob-ntsc
• preserves PAL as the default for backwards compatibility
• selects NTSC as 720x480 when explicitly requested
• rejects conflicting PAL+NTSC flags
• propagates the resolved video standard through ConversionOptions into VobSubExtractor
• documents both flags in the CLI help
• adds regressions for default PAL, explicit PAL, explicit NTSC and conflicting flags

Current-base verification

• this PR is intentionally based on fix/vob-multistream-overwrite-fa6d / internal PR Protect multi-stream VOB outputs from accidental overwrite #66 so its diff contains only the video-standard behavior change
• all four files modified by the original internal Allow explicit NTSC for VOB extraction #22 follow-up had unchanged base blobs relative to the new Protect multi-stream VOB outputs from accidental overwrite #66 head
• the reviewed Allow explicit NTSC for VOB extraction #22 blobs were therefore ported exactly, with no conflict resolution or unrelated edits
• current diff is one commit touching only ConvertCommand.cs, SubtitleConverter.cs, HelpDisplay.cs, and VobSubExtractorTest.cs
• full solution restore passed
• full solution build passed
• full solution test suite passed on the first run
• retry step was not used
• validation run: #34725536557

Dependency: internal PR #66 (validated; keep both drafts unmerged).

This PR supersedes internal draft #22.

AI assistance: ChatGPT was used to independently re-audit the PAL/NTSC limitation, verify exact base-file identity against the validated current overwrite fix, and port the isolated follow-up with its focused regressions.