Expand HtmlTextWriter enum coverage for HTML5 (P3)

[csharpfritz]

Summary

The HtmlTextWriter enums (HtmlTextWriterTag, HtmlTextWriterAttribute, HtmlTextWriterStyle) mirror .NET Framework 2.0 era HTML. Modern custom controls need HTML5 semantic elements, ARIA attributes, and CSS3 properties.

Missing Elements

HtmlTextWriterTag

Nav, Section, Article, Header, Footer, Main, Figure, Figcaption, Details, Summary, Dialog, Template

HtmlTextWriterAttribute

• data-* attributes (needs pattern support or common data attributes)
• aria-* attributes (aria-label, aria-hidden, aria-expanded, aria-describedby)
• role, placeholder, required, autofocus, pattern, min, max, step

HtmlTextWriterStyle

• Flexbox: flex-direction, justify-content, align-items, flex-wrap, gap
• Grid: grid-template-columns, grid-template-rows, grid-gap
• Visual: transform, transition, animation, opacity, box-shadow, border-radius

Use Case

DepartmentPortal controls use <nav>, <section>, <article>, data-* attributes, aria-label for accessibility, and flexbox/grid layouts. Developers must currently use string literals instead of the type-safe enum API.

Priority

P3 - Enables modern HTML5/CSS3 custom controls without resorting to string literals.

[csharpfritz]

Implemented in commit 48dd5fb on feature/ascx-sample-milestone branch.

What was built:
HtmlTextWriter enum expansion for extended HTML tag and attribute support.

Test coverage: New bUnit tests added
Dev docs: See dev-docs/proposals/p1-p5-custom-controls-framework.md