[Image] Add Image component

[seb-jean]

Q	A
Bug fix?	no
New feature?	yes
Deprecations?	no
Documentation?	yes
Issues	Fix #3188 / #3549
License	MIT

This PR adds a new symfony/ux-image package providing Twig components for rendering <img>, <picture> and <source> HTML elements.

Components

• <twig:ux:img> : renders an <img> element (runtime renderer, void element)
• <twig:ux:picture> : renders a <picture> wrapper (template-based, accepts children)
• <twig:ux:source> : renders a <source> element (runtime renderer, void element)

Twig functions

• ux_img() : programmatic alternative to the component
• ux_picture() : programmatic alternative with sources array

Supported attributes

All standard HTML attributes: src, alt, srcset, sizes, width, height, loading, decoding, fetchpriority, crossorigin, referrerpolicy, ismap, usemap. Extra attributes (class, id, data-*, …) are passed through.

srcset and sizes accept both strings and PHP arrays for readability.

Validation

The components validate attributes against the HTML specification at render time:

• alt is always required (empty string allowed for decorative images)
• src or srcset must be provided
• srcset must not mix width (w) and density (x) descriptors
• srcset must not contain duplicate descriptors
• sizes requires all srcset candidates to use w descriptors
• sizes="auto" (or "auto, <fallback>") requires loading="lazy"
• Enum attributes (loading, decoding, fetchpriority, crossorigin, referrerpolicy) are validated
• <source> does not accept src

Example

{# Basic image #}
<twig:ux:img src="/photo.jpg" alt="A photo" width="800" height="600" loading="lazy" />

{# Responsive with srcset #}
<twig:ux:img
    src="/photo-800.jpg"
    alt="Hero"
    :srcset="['/photo-480.jpg 480w', '/photo-800.jpg 800w', '/photo-1200.jpg 1200w']"
    :sizes="['(max-width: 600px) 480px', '800px']"
/>

{# Format selection: AVIF → WebP → JPEG #}
<twig:ux:picture>
    <twig:ux:source srcset="/photo.avif" type="image/avif" />
    <twig:ux:source srcset="/photo.webp" type="image/webp" />
    <twig:ux:img src="/photo.jpg" alt="A photo" width="800" height="600" />
</twig:ux:picture>