fix: consumer-configurable sidebar package selector

[Shewart]

Summary

Fixes the first entry in docs/SHELLUI_DOGFOOD_FIXES.md — the hardcoded ShellDocs-only package list in the sidebar's package selector.

The bug

PackageSelector.razor shipped a static readonly List<Package> hardcoded to ShellDocs' own five packages (ShellDocs, ShellDocs.Markdown, ShellDocs.Core, ShellDocs.CLI, ShellDocs.Components). Every consumer's sidebar surfaced that list — a ShellUI docs site rendered ShellDocs' package names, a MudBlazor site would have too. There was no consumer-config path, and the selector always rendered even for consumers with a single package family (dropdown of one).

Discovered during the first render of shellui-docs while scaffolding it via ShellDocs.

The fix

New public API on ShellDocsOptions

public record DocsPackage(string Id, string Title, string Description, string RootUrl, string? IconPath = null);

public List<DocsPackage> Packages { get; }
public ShellDocsOptions AddPackage(string id, string title, string description, string rootUrl, string? iconPath = null);

Consumers declare their family in Program.cs:

builder.Services.AddShellDocs(o =>
{
    o.AddPackage("shellui",             "ShellUI",             "The component library.",  "/docs/introduction");
    o.AddPackage("shellui.components",  "ShellUI.Components",  "Widgets and primitives.", "/docs/components");
    o.AddPackage("shellui.cli",         "ShellUI.CLI",         "`shellui add`, config.",  "/docs/cli");
});

PackageSelector.razor behavior changes

• Injects ShellDocsOptions; reads Options.Packages at render time; hardcoded ShellDocs list deleted.
• Early-returns rendering nothing at all when Options.Packages.Count <= 1. A consumer with zero or one packages sees no selector — no dropdown-of-one noise.
• Selected-package resolution now does longest-prefix matching on the current URL against each package's RootUrl. Previously used a fuzzy substring match that wrongly picked ShellDocs for /docs/components/callout when it should have picked ShellDocs.Components.
• Default box glyph used when a package's AddPackage(...) call omits an iconPath — same 24×24 stroke contract as consumer-supplied paths.

Preview app migration

examples/ShellDocs.Preview/Program.cs now calls AddPackage(...) for its five packages. Proves the consumer flow end-to-end and keeps the preview app's selector rendering identically to before.

Test plan

• dotnet build shelldocs.slnx — clean, 0 warnings, 0 errors
• dotnet test shelldocs.slnx — 121 / 121 passing (+2 new in PackageRegistrationTests)
• Preview app smoke:
  • /docs/introduction → selector shows ShellDocs (correct)
  • /docs/markdown → ShellDocs.Markdown (correct)
  • /docs/components/callout → ShellDocs.Components (previously would have wrongly selected ShellDocs under the old substring match)
• Empty-packages case (verified via unit test): new ShellDocsOptions().Packages is empty by default; PackageSelector early-return path renders no markup

Files touched

• src/ShellDocs.Components/ShellDocsOptions.cs — new DocsPackage public record, Packages list, AddPackage fluent
• src/ShellDocs.Components/Chrome/PackageSelector.razor — rewritten; drops hardcoded list, injects options, early-returns, longest-prefix match
• examples/ShellDocs.Preview/Program.cs — migrated to AddPackage(...)
• docs/SHELLUI_DOGFOOD_FIXES.md — entry moved from Open to Fixed
• tests/ShellDocs.Tests/PackageRegistrationTests.cs (new, 2 tests)