text: Support HTML <mark> tag with highlight color

[ylinwind]

Description

Adds support for the HTML <mark> tag in the text/HTML renderer, rendering marked text with a background highlight.

The highlight color is customizable via the color attribute or the background-color style declaration, parsed by the existing try_parse_color helper:

<mark>highlighted</mark>                                   <!-- default light yellow -->
<mark color="blue-200">blue tint</mark>                   <!-- Tailwind scale -->
<mark color="amber/30">amber, 30% opacity</mark>          <!-- opacity modifier -->
<mark style="background-color: #fca5a5">custom hex</mark>  <!-- hex -->

• Accepts hex (#3366ff), named colors (blue), Tailwind scales (blue-200), and opacity (amber/30).
• Defaults to a light yellow highlight when no color is given.

TextMark gains an additive highlight: Option<Hsla> field (private module, not part of the public API). The <mark> demos were added to the HTML example fixture, with a unit test covering the tag.

Note: AI-generated code, refactored to match project style.