Warn when a filter or output setting reduces bit depth on high-bit-depth sources

[StuartCameronCode]

Follow-up to #46. Adds inline advisory banners so users know when enabling a pass — or picking an output option — will silently reduce colour precision on a >8-bit source (e.g. 10-bit ProRes 422).

Cases covered

• Filter pass (input side): the filter schema gains an optional maxBitDepth. When set and the source exceeds it, the worker down-converts to that depth for the pass and restores it afterward (a lossy round-trip), so the pass settings panel shows a warning. DeScratch is the only such filter today (8-bit only, YV12/YV16/YV24), so its schema declares maxBitDepth: 8. IVTC/VFM is lossless now (clip2, Fix IVTC failure on >8-bit sources (e.g. 10-bit ProRes 422) #46); f3kdb/deblock/cas all handle 8–16 bit natively.
• Output side: choosing a chroma-subsampling conversion (4:2:0 / 4:2:2) forces an 8-bit output format even for a 10-bit source, so the encoding settings show a warning next to the dropdown. "Original" preserves the source's bit depth (only HuffYUV forces a format otherwise, and the software/ProRes/lossless encoders keep the pipeline depth).

Implementation

• utils/pixel_format.dart
  • pixelFormatBitDepth() — parses ffprobe pix_fmt strings (yuv422p10le → 10, p010le → 10, rgb48le → 16, unknown → 8 so no spurious warnings).
  • filterBitDepthWarning() / chromaConversionBitDepthWarning() — pure decision helpers returning the message or null.
• widgets/warning_banner.dart — extracted the repeated advisory-banner widget (icon + text in the theme error-container style); the existing OpenCL-unavailable warning now reuses it.
• pass_settings_container.dart / settings_dialog.dart — render a WarningBanner when the respective helper returns a message.

Tests

test/pixel_format_test.dart — 17 unit tests covering the parser (8/10/12/16-bit formats, null/unknown fallback, case/whitespace) and both decision helpers (enabled/disabled, no ceiling, source fits, 8-bit source, unknown format, converting vs Original). The UI widgets are thin wrappers over these tested pure functions.

Generated .g.dart is gitignored and regenerated by CI's build_runner, so only the model/JSON source changes are committed.

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