Frame-accurate preview + seeking (FrameMap frame-mapping seam)

[StuartCameronCode]

Summary

Makes the preview frame-accurate and adds frame-accurate seeking, built on a new FrameMap descriptor that becomes the single source↔output frame-mapping seam in the worker.

FrameMap (worker/src/models/processing_pipeline.rs)

• enum FrameMap { Identity, Fanout, Decimate, Retime } with output_count() (forward) and inverse() (carrying an exact flag).
• ProcessingPipeline::{frame_maps, output_count, invert, total_radius} compose the per-pass maps. Today: QTGMC double-rate → Fanout{2}; IVTC/soft-telecine → Decimate{cycle, keep}; everything else identity.
• Replaces the hardcoded is_double_rate / is_ivtc progress ladder in pipeline_executor.rs with pipeline.output_count(vspipe_total) — behavior-preserving (proven by test).
• A future frame-rate filter plugs in as a Retime variant with no new special-cases.

Frame-accurate preview (worker + templates)

• generate_preview takes an integer source frame, decodes the window [target-radius ..= target+radius] (radius = total_radius(), floored at MIN_PREVIEW_RADIUS), and emits the exact output index output_count(local) — replacing the old clip[num_frames // 2] "middle of 11" heuristic.
• Seeks land exactly via center-of-interval (frame - 0.5)/fps (with -ss before -i), avoiding ±1 PTS-rounding ambiguity. The same fix makes start-frame trimming exact. Window geometry is the pure, unit-tested preview_window().
• Echoes PREVIEW_INFO:frame=N back. Stays inside the pipe_source streaming model — no BestSource reintroduced.

Flutter

• Preview driven by an integer frame index end-to-end (no time round-trip). Before/after now seek to the same source frame.
• New FrameMath (lib/services/frame_math.dart) — pure, stable position⇄frame conversion.
• View-model exposes currentFrameIndex / totalFrames / seekToFrame / stepFrame; the preview panel gains ◀/▶ step-by-frame buttons + a f N / total readout.
• generateProcessedPreview now guards against 0×0 (probe-pending) jobs — a slow first-run ffprobe no longer surfaces a misleading "Invalid video dimensions" error; it self-heals once probing completes.

Testing

Suite	Result
cargo test --lib	75 pass (incl. FrameMap + preview_window edge cases)
cargo test --test filter_integration_test	56 pass (incl. new test_55_preview_selects_exact_frame, test_56_frame_count_mapping)
flutter test test/frame_math_test.dart	5 pass (incl. round-trip-no-drift)
Flutter pure unit tests	62 pass
flutter analyze (changed files)	clean

Also built and launched the macOS debug app.

Notes

• Docs: added a processing_pipeline.rs row to the CLAUDE.md worker key-files table.
• Not in this PR: warming bundled ffmpeg/ffprobe after deps download (a separate macOS-beta first-run Gatekeeper-assessment mitigation).

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