Crop & Resize: Add Borders to a fixed canvas, in a chosen colour (#86)

[StuartCameronCode]

Closes #86.

What users get

A new Borders section in Crop & Resize:

• Add Borders pads the picture to a fixed canvas without rescaling it, independent of Resize — crop a dirty edge, border back out to the authoring size.
• Canvas Preset in simple mode (PAL DVD 720×576, NTSC DVD 720×480, 720p, 1080p); exact width/height in advanced mode.
• Border Colour: black / grey / white / custom #RRGGBB, also used by the existing Pad to Fill.
• A picture larger than the canvas fails the job with a readable message instead of silently producing the wrong frame size.
• Stamped sinceAppVersion: 1.2.0.

How

• One {{#BORDERS}} step at the very end of both templates, after grain and after CHROMA_CONVERT, so nothing noises or resamples the bars. Pad to Fill's block now only records its box and uses the same step.
• _border_color() converts the chosen RGB into the clip's own format via resize with the source's matrix/range (ColorMetadata::zimg_matrix / is_full_range).
• Bug fix: AddBorders with no color= fills luma 0 — below video black on limited range. Pad to Fill's bars were therefore "blacker than black"; they are now 16 (8-bit) / 64 (10-bit).
• Offsets stay on the chroma grid, and on the field grid when the picture is still interlaced (deinterlacing off + detected field order).

Tests

• Rust: model unit tests; test_156–test_160 (canvas, bars-last ordering, matrix/range, field alignment, Pad to Fill sharing the step).
• Flutter: converter round trip; script-gen test via the panel's dynamic values (push gate); new heavy integration_borders_test.dart — bar levels at 8-bit 4:2:0 and 10-bit 4:2:2, custom white = 940 at 10-bit, interior bit-identical positioning, oversize error, preview.
• Mutation check: removing color= makes the heavy tests fail (luma 0 vs 16/64/940).
• Locally (macOS arm64): cargo test green apart from the whisper test (add-on not installed); flutter test --exclude-tags heavy green; heavy aspect/upscale/borders suites green.

Found along the way, not fixed here

• FFmpeg 9.0.1 converts pixels (not just tags) when -colorspace bt709 etc. is stamped on the untagged Y4M pipe — every tagged encode is colour-shifted.
• Clean-aperture MOVs: ffmpeg applies the clap crop (720→702) and the decoder's -s 720x576 rescales it back.

Both are being handled on separate branches.

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