Protect multi-stream VOB outputs from accidental overwrite

[Blackspirits]

Summary

The VOB batch preflight currently protects only the unnumbered base output path. Multi-stream extraction writes numbered pairs such as movie.0.sub/.idx and movie.1.sub/.idx, so an existing numbered output can be overwritten even when --overwrite is not supplied.

This change:

• moves overwrite protection into VobSubExtractor, after the actual subtitle-stream count and output names are known
• computes the exact output paths before any stream is written
• checks both .sub and companion .idx for every resolved output
• preserves --overwrite behavior
• removes the misleading best-effort base-path preflight from SubtitleConverter
• keeps an unused unnumbered base path from blocking a multi-stream extraction
• adds regressions for numbered-output protection and unused-base-path behavior

Current-base verification

• the defect is still present on upstream fa6d8623c2adb0c646b6d31a11b221b855339bbe
• all three affected base files are byte-for-byte identical to the files reviewed for internal PR Protect multi-stream VOB outputs from accidental overwrite #17
• the reviewed three-file fix was therefore ported exactly onto the current upstream tree, with no conflict resolution or unrelated edits
• current diff is one commit touching only SubtitleConverter.cs, VobSubExtractor.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: #34725292765

Base is current upstream fa6d8623c2adb0c646b6d31a11b221b855339bbe.

This PR supersedes internal draft #17.

AI assistance: ChatGPT was used to independently re-audit the current VOB multi-stream overwrite path, verify that the affected base files are unchanged from the reviewed draft, port the exact fix onto current upstream, and preserve the focused regressions.

[Blackspirits]

Independent adversarial re-check on current upstream base: confirmed the current CLI still preflights only the unnumbered VOB base path while multi-stream extraction writes numbered pairs. The fix computes actual output names after stream discovery and checks every .sub/.idx pair before any write; --overwrite remains unchanged and an unused base path no longer blocks multi-stream output. Full current-base CI run #34725292765 passed restore/build/full tests on the first run with no retry. No blocker identified. Keep draft; no merge/promotion performed.