feat: ShellIcons.Avalonia preview and a shared XAML generator

[Shewart]

Summary

Avalonia target (preview, not published)

• New ShellIcons.Avalonia package (net8.0, Avalonia 12.1): typed controls (<icons:ChevronRight />), an Icon dispatcher (<icons:Icon Kind="Search" />) and the same IconName / IconCatalog / IconInfo API as the MAUI package. xmlns https://shellicons.dev/avalonia.
• Drawn directly: each icon is one custom-drawn control. The path data is converted at build time and parsed once per icon into a cached StreamGeometry.
• Colored like text: Foreground is the inherited text foreground, so an icon in a Button takes the button's color, like currentColor in the browser.
• Kind, not Name: Avalonia's Name is the element name, so the dispatcher property is Kind.
• Accessibility: Title sets the automation name and exposes the icon as an image; without it the icon is hidden from screen readers.

One generator for MAUI and Avalonia

• Target picked per project: ShellIcons.Generator.Xaml now has one catalog reader and one emitter. The consuming project picks its target with <ShellIconsTarget>Maui|Avalonia</ShellIconsTarget>; before this it emitted MAUI code into any project that referenced it.
• IconCatalog is generated: IconCatalog and IconInfo are now generated for both targets, replacing MAUI's hand-written IconCatalog.cs. The API is unchanged.
• New diagnostic: SHELLICONS006 warns when ShellIconsTarget is missing.

Docs

• Avalonia section on the docs site (getting started, names and catalog), plus updates to the README, CHANGELOG, RELEASING and the custom-icon contract.
• ShellDocs 0.1.11-alpha for the docs site (was 0.1.7). No code changes were needed; outdated workaround comments were trimmed.

CI

• Avalonia in the main solution: the Avalonia projects are in ShellIcons.slnx, which needs no workloads, and ci.yml smoke-packs ShellIcons.Avalonia.

Breaking changes

None for published packages. In the unpublished MAUI preview, IconCatalog is no longer a partial class.

Test plan

• ShellIcons.slnx builds with 0 warnings; 134 tests pass (57 generator, 44 Blazor, 33 Avalonia)
• ShellIcons.Maui.slnx builds all 5 targets with 0 warnings; 34 MAUI tests pass
• Each commit builds on its own
• Avalonia tests render through Skia headless: every icon parses, stays on the 24×24 grid and draws inside its own cell; size, stroke, color, XAML namespace and accessibility checked
• Rendered contact sheet and full-catalog image reviewed by eye
• dotnet pack of ShellIcons.Avalonia produces a clean package (icon, readme, no analyzer dependency)
• All 17 docs pages render on ShellDocs 0.1.11; theme toggle, Preview/Code tabs and icon search work; shelldocs build succeeds
• Avalonia tests pass on the ubuntu CI runner (only run on Windows so far)
• Avalonia checked in a real desktop app

Follow-ups

• Release 0.2.0-alpha: bump <Version>, decide whether to publish ShellIcons.Avalonia (add it to release.yml; it packs on ubuntu)
• Per-package readmes before publishing MAUI or Avalonia
• Update the Lucide catalog (pinned at 0.475.0; Lucide is at 1.59.0)
• Retire the GitHub Pages docs workflow; the site now runs as a container