fix(php-transformer): fit slideshow slides into the recovered stage box

[chubes4]

Problem

#1858 recovered the stage box a slideshow framed its slides into, but inferred the wrong fit. It assumed each centered layer covered the box, so it took the smallest layer on each axis and cropped with object-fit: cover.

Measuring the source directly shows the opposite. For a stage whose real box is 885x590:

slide	natural	measured layer	contain predicts	cover predicts
portrait	729x1100	393x593	391x590	885x1335
landscape	1100x729	886x587	885x587	890x590

The layers are fitted inside the box and centred — a portrait slide is letterboxed, not cropped. So the box is the largest layer on each axis, not the smallest: 886x592 against a measured 885x590, inside half a percent.

The shipped inference produced a portrait 393/588 frame that cropped every landscape slide, and the stage also stretched to the full content column instead of its authored width.

Change

• Reconstruct the box from the largest centered layer on each axis.
• Fit slides with object-fit: contain, centred, so any orientation keeps one steady frame.
• Bound the stage to its authored width (stageMaxWidth) and centre it, instead of filling the column.
• An explicitly declared pixel height still wins and keeps its existing cover behaviour.

Test

php tests/unit/authored-carousel-block.php covers the recovered ratio and width, the emitted custom properties, the fitting stylesheet, declared-pixel-height precedence, and the uncentered-offset rejection. Full composer test passes locally, including all 311 parity fixtures.

AI assistance disclosure

Implemented with AI assistance — grok-4.6 via OpenCode. The correction came from measuring the live source's clip box and per-slide layer boxes against both fit models. A human retains review and merge authority.