Flex text+image rows become core/media-text and lose authored column sizing, gap and the visual wrapper #3596

Problem

A flex row of a text column and an image column becomes core/media-text, which discards the row's authored geometry. Media & Text applies its own grid (1fr 50%) and pads its content column (0 8%). The source flex ratios, gap and the text column's max-width no longer decide the layout, so the text wraps onto extra lines and the section changes height.

Neutral reproduction on trunk (HtmlTransformer::transform), measured at 1440px with core's block-library CSS:

<style>
body{margin:0}
.row{max-width:1200px;margin:0 auto;display:flex;align-items:center;justify-content:space-between;gap:80px}
.copy{flex:1 1 0%;max-width:480px}
.copy h2{font-size:46px;line-height:1.12;margin:0 0 25px}
.visual{flex:1.2 1 0%;display:flex;justify-content:center;align-items:center}
.visual img{width:100%;max-width:780px;height:auto;display:block}
</style>
<div class="row"><div class="copy"><h2>Complete care for your computer in one click</h2><p>A short paragraph that explains the product in a couple of lines of body copy.</p></div><div class="visual"><img src="shot.png" width="1600" height="1000" alt="Dashboard"></div></div>

Emitted blocks: media-text > group, heading, paragraph.

Real case: on assistouest.fr /logiciel-maintenance-informatique/ (.asst_main_container), the text column renders 354px instead of 480px, and the heading wraps from 2 lines to 3. The image wrapper .asst_main_visual is also lost: it carried perspective and a ::before glow behind the image.

This is the image/flex sibling of #3583 (text + video grid, fixed by #3586) and matches #2473's acceptance ("select a native group/image composition when core/media-text cannot preserve the source contract").

Expected

• Promote to core/media-text only when its geometry reproduces the source row.
• Otherwise, emit a native group (flex or grid layout) containing the text group and an image block inside the retained visual wrapper. Source column sizing, gap, alignment and wrapper styles stay authored.
• Rows that Media & Text does reproduce keep converting to it.

Acceptance

• A regression with the fixture above fails on trunk. It checks h2 and img boxes within 1px of the source at 1440px, plus the 390px stacked layout if the fixture adds a breakpoint.
• The existing media-text fixtures and contract tests pass. That includes Preserve native text/video grids and editable Media Library videos #3586's video-grid-layout test.
• Block markup stays valid (no recovery prompt).