fix(codegen): avoid cropping exported image fills twice

[owjs3901]

about-422-3376 was the worst screen in the corpus at 7.46%, and its page height was exactly right - 7240 against a design height of 7240. Every defect fixed here so far has been vertical. This one was not: the content was mis-composed inside a correct height.

The cause is a double crop.

A CROP image fill carries its framing as a matrix over the image's own 0..1 space, and the generator read that matrix and turned it into a CSS background-position/background-size. That would be correct if the file being positioned were the original image. It is not. image_fill_source names a node.exportAsync rendition, and Figma has already applied imageTransform when producing those pixels. Applying the matrix again framed an already-framed picture.

That also explains why the earlier attempt to honour the crop looked like a fix - the symptom was real, the photographs genuinely were wrong - while measuring worse.

A cropped fill now maps its exported frame once onto its layout box. FIT, FILL and TILE are unchanged, and image_crop is deleted rather than left for someone to reach for again.

Screen	Width	Before	After
about-422-3376	360	7.46%	5.51%
about-422-3180	992	4.06%	3.01%
about-422-2987	1920	2.41%	1.77%

All three widths improve, page heights unchanged, and the other twelve screens are byte-identical.

Verification

Measured independently in the main checkout with binaries built from main and from this branch, full 15-screen acquire and render on each side. My figures match the worker's to the decimal. The baseline run also fails correctly against the new thresholds (7.46% > 5.51%), which confirms the two runs really are measuring different code rather than reusing inputs.

The change is a net deletion. No viewport, breakpoint, node id or capture-specific constant - I grepped the added lines. All 268 plugin byte-parity goldens unchanged. Korean wordBreak: keep-all untouched. Only the three measured about thresholds lowered.

The responsive_screen expectation that asserted the old doubled crop (53.51% -25.92%/91.26% 104.43%) is corrected, with a comment stating why the export is mapped once.

Gates: fmt 0, clippy --locked --workspace --all-targets --all-features -D warnings 0, cargo test --workspace 1070 passed / 0 failed / 2 ignored, insta --check no drift.