Fluent2: add README with migration notes and compatibility contract

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/Avalonia.Themes.Fluent2/README.md

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+# Avalonia.Themes.Fluent2
+
+A modern Fluent theme for Avalonia implementing the **Fluent 2 / WinUI 3 visual
+language** (the design system Microsoft shipped with Windows 11), built as a
+**drop-in replacement** for `Avalonia.Themes.Fluent`.
+
+Reference implementation: the WinUI 3 resource dictionaries (release 1.5.2
+baseline, individual controls up to 1.8.x) as ported by Uno Platform's
+`Uno.UI.FluentTheme.v2`.
+
+## Usage
+
+```xml
+<Application.Styles>
+    <Fluent2Theme />
+</Application.Styles>
+```
+
+Migrating from `Avalonia.Themes.Fluent`: change the package reference and swap
+`<FluentTheme />` for `<Fluent2Theme />`. Everything else keeps working:
+
+- **Resource key overrides** — every v1 resource key (`ButtonBackground`,
+  `TextControlBorderBrushFocused`, `ControlCornerRadius`, …) still resolves
+  with a compatible runtime type. Values changed; names did not.
+- **`Classes="accent"`**, pseudo-class selectors, template part names and named
+  control themes are unchanged.
+- **`Fluent2Theme.Palettes`** keeps the v1 `ColorPaletteResources` API. `Accent`
+  flows into all the new accent tokens. Non-accent legacy colors
+  (`RegionColor`, `BaseHigh`, `BaseMediumHigh`, `BaseMedium`, `BaseLow`,
+  `ChromeMedium`, `ErrorText`) heuristically drive the nearest Fluent 2 tokens;
+  for exact control override the token keys directly
+  (`SolidBackgroundFillColorBase`, `TextFillColorPrimary`, …).
+- **`DensityStyle="Compact"`** is preserved and re-derived against the new
+  metrics.
+
+## What changed visually (intentional)
+
+| Default | v1 | Fluent2 |
+|---|---|---|
+| `ControlCornerRadius` | 3 | **4** |
+| `OverlayCornerRadius` | 5 | **8** |
+| `ButtonPadding` | 8,5,8,6 | **11,5,11,6** |
+| `TextControlThemePadding` | 10,6,6,5 | **10,5,6,6** |
+| Focused text border | 2 uniform | **1,1,1,2** + accent underline |
+| Button press | scale(0.98) | **fill change + 83 ms cross-fade** (WinUI has no press scale) |
+| Control borders | flat | **gradient elevation borders** (darker bottom edge) |
+| List/tree/combo selection | full-row accent | **subtle fill + 3×16 accent pill** |
+| Scroll bars | 16 px rail | **12 px rail, 2 px collapsed thumb** |
+| Window background | AltHigh | **SolidBackgroundFillColorBase** (#F3F3F3/#202020) |
+| Default accent shades | HSL-computed | **WinUI static values** (when no OS accent) |
+
+The full WinUI 3 token family (`TextFillColor*`, `ControlFillColor*`,
+`SubtleFillColor*`, `ControlStrokeColor*`, `CardBackgroundFillColor*`,
+`SolidBackgroundFillColor*`, `SystemFillColor*`, `AccentFillColor*`, elevation
+border brushes, acrylic fallbacks) is available for app use alongside the
+legacy `SystemControl*` aliases.
+
+## Known scope limitations
+
+- Acrylic/Mica surfaces ship as their solid fallback colors (WinUI's own
+  fallback mode); real translucency is a possible future enhancement.
+- `TabControl` maps to the WinUI **Pivot** header look (Avalonia's TabItem
+  semantics); the TabView document-tab look is out of scope.
+- No HighContrast variant yet (same as v1).
+- AnimatedIcon glyph animations are approximated with static glyphs and simple
+  transitions.
+- OS-provided accent colors still use HSL-computed shades, which can deviate
+  slightly from Windows' palette algorithm.
+
+## Compatibility tests
+
+`tests/Avalonia.Themes.Fluent2.UnitTests` enforces the drop-in contract against
+the live v1 theme: key parity in both variants with compatible runtime types,
+implicit `ControlTheme` coverage, palette semantics, compact-density key
+parity, and an instantiation/render smoke test over every themed control.