chore: Upgrade Avalonia to 12.0 and migrate codebase

[yuto-trd]

Description

• Upgrade Avalonia from 11.x to 12.0 and update related dependencies across the solution (Directory.Packages.props, csproj files).
• Migrate codebase to Avalonia 12 APIs: ReflectionBinding for dynamic bindings, x:DataType for compiled bindings, PlaceholderText, FocusChangedEventArgs, Math.Clamp, Avalonia.Input.Platform namespace, logical-tree traversal, and TopLevel-based scaling.
• Replace Avalonia.Diagnostics with AvaloniaUI.DiagnosticsSupport (WithDeveloperTools()), remove the custom BcTabItem/BcTabView/DirectoryTreeView controls, and migrate UI to FluentAvalonia (FA) components including the renamed Beutl.FluentIcons.FluentAvalonia package.
• Drop the obsolete DirectoryViewTest project and adjust several window/editor views (styles, drag handling, IListEditorViewModel.IsEnabled, etc.) for consistency under the new framework.

Breaking changes

• Extensions depending on the custom BcTabItem/BcTabView or DirectoryTreeView controls must migrate to the FluentAvalonia equivalents, since these controls were removed.
• Extensions referencing the FluentIcons.FluentAvalonia package must switch to Beutl.FluentIcons.FluentAvalonia.
• Targeting Avalonia 12 means extensions must be recompiled against the new Avalonia 12 API surface.

Fixed issues

None

[github-actions]

The following TODO comments were found:

beutl/src/Beutl/ViewModels/Editors/CoreObjectEditorViewModel.cs

Line 134 in bc934bb

// TODO: EngineObject.IsEnabledと同期する

[github-actions]

Package	Line Rate	Branch Rate	Complexity	Health
Beutl.ProjectSystem	17%	10%	926	➖
Beutl.Utilities	73%	67%	358	➖
Beutl.Language	1%	50%	1148	➖
Beutl.Configuration	40%	20%	314	➖
Beutl.Threading	100%	93%	122	✔
Beutl.NodeGraph	6%	2%	2490	➖
Beutl.Core	39%	35%	3070	➖
Beutl.Engine	20%	18%	16812	➖
Beutl.Extensibility	11%	14%	111	➖
Beutl.Editor	70%	65%	1011	➖
Summary	23% (14513 / 62203)	22% (4086 / 18627)	26362	➖

Minimum allowed line rate is 0%