IVTC: fallback deinterlace for leftover combing

[StuartCameronCode]

Summary

Implements the feature scoped earlier for a user bug report: an IVTC source with a broken 3:2 cadence at scene changes (and some field-blended dissolves) leaves residual combing after VFM/VDecimate alone, because those frames simply couldn't be cleanly field-matched.

• New opt-in ivtcFallbackDeinterlace toggle (IVTC method only, default off): patches any frame VFM flagged as still combed with a QTGMC deinterlace of that same frame, instead of leaving it combed.
• New ivtcFallbackPreset (default Fast, not the general Slower QTGMC default — this runs a full second QTGMC pass on top of an already-slow IVTC pass).
• Applies to both the encode and preview scripts, so the preview shows what the real output will look like.

Mechanism

Before wiring anything up, I probed the actual bundled vivtc plugin (not assumed from memory or docs) to confirm the two behaviors this depends on:

• VFM stamps a _Combed frame property on every frame — already true today, just unused elsewhere.
• VDecimate accepts a clip2 kwarg: decimation decisions come from clip, output frames come from clip2. VFM itself already uses this exact idiom (its own 8-bit-metrics / full-depth-output split), so this isn't a new pattern for this codebase.

Script shape: VFM runs unchanged → a parallel QTGMC pass deinterlaces the full source at single rate → FrameEval builds a hybrid clip that swaps in the QTGMC frame wherever _Combed is set → VDecimate(clip, clip2=hybrid, ...) keeps its drop timing from the plain VFM clip but takes output frames from the hybrid. The preview template has no VDecimate call (single-frame), so the hybrid clip replaces clip directly there instead.

With the toggle off (default), the generated script is byte-for-byte what it was before this feature existed.

Test plan

• Probed the real bundled vivtc.dylib directly via vspipe (not just documentation) to confirm VDecimate's clip2 parameter and VFM's _Combed property both exist as expected
• cargo test — full suite green (161/161 in filter_integration_test.rs; the one pre-existing failure elsewhere, test_subtitle_generation_srt_file, is a local missing-whisper-addon environment gap, unrelated)
• 4 new Rust tests: fallback script shape, custom preset/TFF, preview-path parity, and the off-by-default byte-identical guard
• 2 new Flutter script-generation tests in integration_filter_parameters_test.dart
• flutter analyze clean on every touched file
• flutter test --exclude-tags heavy — 942 passed, 0 failed

Base branch is update-tracking (this branches off it, per the parent conversation) rather than main — rebase onto main once that one merges.

🤖 Generated with Claude Code

https://claude.ai/code/session_014GLXdGLfPwgYjW1AkonGqN