Detect interlaced sources from frame flags

[bjay-wk]

Summary

• Update local encoding interlace detection to check stream field_order first and fall back to sampled frame-level interlaced_frame flags.
• Keep existing downscale behavior: when input is detected as interlaced, CLI applies bwdif=0:-1:0 before scaling.
• Add unit coverage for field_order, frame-flag fallback, and progressive metadata.

Validation

• cargo test media::transcode::tests
• Sample MXF reports field_order=tt, so the CLI classifies it as interlaced and applies bwdif during local downscale.