feat: FlexCrop clip edges in composition pixels

[J160KU]

Summary

• FlexCrop: Kdenlive-style Crop Left / Right / Top / Bottom under Clip properties → Basic → Transform (below Corner). Cropped pixels are fully transparent.
• Inspector and edit_item transform.crop / transform.flexCrop use composition pixels (left/right 0…canvas width, top/bottom 0…canvas height). Storage stays a 0–1 canvas fraction. transform.crop: null clears crop.
• Documents the feature in CHANGELOG.md [Unreleased], a Clip crop section at the top of README.md / README_ZH.md, and the generated agent tool catalog.

Test plan

• Select a video or image clip → Clip properties → Basic → Transform → Crop Left/Right/Top/Bottom; confirm values are px and the preview checkerboard shows through cropped edges.
• Drag opposite edges toward each other and confirm a minimum remaining span (clip does not invert).
• Agent / MCP: edit_item with transform.crop: { left: 240 } (pixels, not percent); transform.crop: null clears.
• tsx src/editor/clipCrop.verify.ts, tsx src/agent/tools/edit-item-generic.verify.ts, tsx src/components/inspector/inspector-controls.verify.ts

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