docs(demo): add a MudBlazor / Fluent UI adapter selector to the top bar (#490)

[phmatray]

Implements #490.

Closes #490.

Executing the implementation plan task-by-task; the checklist below — and the plan on the issue — are
ticked as each task lands. Opened as a draft — will be marked ready after the final task and a
code-review pass.

Plan

• Task 1: Fluent brand tokens with an AA contrast guard
• Task 2: Pre-boot attribute and the AdapterPreference service
• Task 3: The top-bar selector and the honesty note
• Task 4: Home install line and setup steps follow the adapter
• Task 5: Getting Started carries both adapters' setup
• Task 6: Full verification pass

What shipped

• Selector: a MudBlazor | Fluent UI segmented control (aria-pressed buttons in a labelled group) in the top bar. It is persisted in localStorage["fc-adapter"] through window.fcAdapter, and every access is in try/catch. A scoped AdapterPreference service shares it app-wide.
• Accent: :root[data-adapter="fluentui"] in tokens.css overrides all eight accent tokens with Fluent 2's brand ramp (the Spec's starting values, all AA). An inline pre-boot script in index.html sets the attribute, so the first frame is never violet.
• Snippets:
  • The Home hero install line (and its copy button) plus setup step 2 switch to the Fluent UI package and registration.
  • getting-started.md wraps Install / Services / References in ::: adapter-mudblazor / ::: adapter-fluentui containers, shown or hidden by CSS.
• Honesty note: under the nav when Fluent UI is selected. The live forms stay MudBlazor.

Deviations from the plan

• demo.css: the plan asked for color-mix(… 25% …), but the original alpha was .3. I used 30%, so the MudBlazor rendering doesn't change.
• Home.razor: each setup step is @key'd on its source, and the page highlights again after an adapter change. code.js rewrites the <code> it highlights, so without the key Blazor could not update step 2's text. The browser pass caught this: step 2 stayed on MudBlazor.
• shell.css: .fc-adapter-note gets width: 100%, because as a flex item of .fc-shell the auto margins shrank it off-centre. .fc-nav .fc-btn--sm gets white-space: nowrap, because at 1440 px the added control made "Get started" wrap.

Verification

• dotnet build -c Release: 0 warnings, 0 errors. dotnet test -c Release: 2150/2150 passed. ./build.sh Format: succeeded.
• DemoAdapterThemeTests (12 tests): the Fluent block covers every accent token and the five AC7 pairs are ≥ 4.5:1. The pre-boot script runs before blazor.webassembly.js, uses try and only accepts the literal 'fluentui'. Getting Started has exactly one closed container per adapter per section, with the real Fluent UI surface.
• Real-browser pass (Playwright + Chrome, 57 checks, all passing) at 1440 and 390 px:
  • AC1: keyboard toggle with a yellow focus ring and aria-pressed; no horizontal scroll at 390 px. The accessible names stay "MudBlazor" / "Fluent UI" while the visible labels are "Mud" / "Fluent".
  • AC2: the accent switches both ways with no reload.
  • AC3: after a reload, the data-adapter attribute and --fc-violet: #0F6CBD are already set at DOMContentLoaded.
  • AC4: the hero copy button copies dotnet add package FormCraft.ForFluentUI, and step 2 shows the Fluent calls.
  • AC5: Getting Started shows only the selected adapter's blocks, and the code blocks still get their copy bar.
  • AC8: with localStorage throwing, the page loads in MudBlazor colours and still switches for the session.
  • AC9: the honesty note is visible, and the /simplified demo still renders MudBlazor components.
  • AC10: no console errors on Home, /simplified or getting-started under either adapter.