feat: Add 3D Projection Gallery sample

Showcases PlaneProjection and Matrix3DProjection rendering on the Skia
targets (new in Uno Platform 6.6, PR unoplatform/uno#22449): an
interactive card driven by rotation sliders plus a depth-preset gallery.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: MartinZikmund

UI/ProjectionGallery/README.md

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+# 3D Projection Gallery
+
+This sample showcases **`PlaneProjection`** and **`Matrix3DProjection`**, which now render on the [Uno Platform](https://platform.uno) **Skia** targets as of **Uno Platform 6.6** ([PR #22449](https://github.com/unoplatform/uno/pull/22449)).
+
+Projections let you rotate and translate any `UIElement` in 3D space — with real perspective — directly from XAML, without writing any custom rendering code. Think flip cards, carousels, cover-flow effects, and subtle depth.
+
+![3D Projection Gallery running on Skia Desktop](doc/assets/ProjectionGallery.png)
+
+The app runs from a single C# + XAML codebase on the Web (WebAssembly) and Desktop (Windows, macOS, Linux) via the Skia renderer.
+
+## Features shown
+
+- **Interactive card** — three sliders drive the `PlaneProjection.RotationX` / `RotationY` / `RotationZ` of a card, so you can spin it in 3D live.
+- **Depth presets** — the same card at fixed angles, making the perspective foreshortening obvious (the far edge shrinks).
+
+## How it works
+
+A `PlaneProjection` is attached to any element through the inherited `UIElement.Projection` property:
+
+```xml
+<Border Width="240" Height="320">
+    <Border.Projection>
+        <PlaneProjection x:Name="HeroProjection"
+                         RotationX="{Binding Value, ElementName=RotX}"
+                         RotationY="{Binding Value, ElementName=RotY}"
+                         RotationZ="{Binding Value, ElementName=RotZ}" />
+    </Border.Projection>
+    <!-- card content -->
+</Border>
+```
+
+The rotation values are two-way bound to `Slider` controls, so moving a slider rotates the card in real time. For full control you can instead use `Matrix3DProjection` and supply your own 4×4 `Matrix3D`.
+
+![The same card at different fixed rotation angles](doc/assets/ProjectionGallery-Presets.png)
+
+## Codebase
+
+* [**MainPage.xaml**](src/ProjectionGallery/MainPage.xaml): the entire UI — the interactive hero card, the rotation sliders, and the depth-preset gallery, all using `PlaneProjection`.
+* [**MainPage.xaml.cs**](src/ProjectionGallery/MainPage.xaml.cs): a small `Reset` handler that returns the sliders to zero.
+
+## What is the Uno Platform
+
+[Uno Platform](https://platform.uno) is an open-source .NET platform for building single-codebase native mobile, web, desktop, and embedded apps quickly.
+For additional information about Uno Platform or if you have any feedback to share, please refer to the [README.md](../../README.md) file in this Samples repository.

UI/ProjectionGallery/doc/assets/ProjectionGallery-Presets.png

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+; This file is for unifying the coding style for different editors and IDEs.
+; More information at http://editorconfig.org
+
+# This file is the top-most EditorConfig file
+root = true
+
+##########################################
+# Common Settings
+##########################################
+
+[*]
+indent_style = space
+end_of_line = crlf
+trim_trailing_whitespace = true
+insert_final_newline = true
+charset = utf-8
+
+##########################################
+# File Extension Settings
+##########################################
+
+[*.{yml,yaml}]
+indent_size = 2
+
+[.vsconfig]
+indent_size = 2
+end_of_line = lf
+
+[*.sln]
+indent_style = tab
+indent_size = 2
+
+[*.{csproj,proj,projitems,shproj}]
+indent_size = 4
+
+[*.{json,slnf}]
+indent_size = 2
+end_of_line = lf
+
+[*.{props,targets}]
+indent_size = 4
+
+[*.xaml]
+indent_size = 4
+charset = utf-8-bom
+
+[*.xml]
+indent_size = 4
+end_of_line = lf
+
+[*.plist]
+indent_size = 4
+indent_style = tab
+end_of_line = lf
+
+[*.manifest]
+indent_size = 4
+
+[*.appxmanifest]
+indent_size = 4
+
+[*.{json,css,webmanifest}]
+indent_size = 2
+end_of_line = lf
+
+[web.config]
+indent_size = 4
+end_of_line = lf
+
+[*.sh]
+indent_size = 2
+end_of_line = lf
+
+[*.cs]
+# EOL should be normalized by Git. See https://github.com/dotnet/format/issues/1099
+end_of_line = unset
+
+# See https://github.com/dotnet/roslyn/issues/20356#issuecomment-310143926
+trim_trailing_whitespace = false
+
+tab_width = 4
+indent_size = 4
+
+# Sort using and Import directives with System.* appearing first
+dotnet_sort_system_directives_first = true
+
+# Avoid "this." and "Me." if not necessary
+dotnet_style_qualification_for_field = false:suggestion
+dotnet_style_qualification_for_property = false:suggestion
+dotnet_style_qualification_for_method = false:suggestion
+dotnet_style_qualification_for_event = false:suggestion
+
+#### Naming styles ####
+
+# Naming rules
+
+dotnet_naming_rule.interface_should_be_begins_with_i.severity = suggestion
+dotnet_naming_rule.interface_should_be_begins_with_i.symbols = interface
+dotnet_naming_rule.interface_should_be_begins_with_i.style = begins_with_i
+
+dotnet_naming_rule.types_should_be_pascal_case.severity = suggestion
+dotnet_naming_rule.types_should_be_pascal_case.symbols = types
+dotnet_naming_rule.types_should_be_pascal_case.style = pascal_case
+
+dotnet_naming_rule.non_field_members_should_be_pascal_case.severity = suggestion
+dotnet_naming_rule.non_field_members_should_be_pascal_case.symbols = non_field_members
+dotnet_naming_rule.non_field_members_should_be_pascal_case.style = pascal_case
+
+# Symbol specifications
+
+dotnet_naming_symbols.interface.applicable_kinds = interface
+dotnet_naming_symbols.interface.applicable_accessibilities = public, internal, private, protected, protected_internal, private_protected
+dotnet_naming_symbols.interface.required_modifiers =
+
+dotnet_naming_symbols.types.applicable_kinds = class, struct, interface, enum
+dotnet_naming_symbols.types.applicable_accessibilities = public, internal, private, protected, protected_internal, private_protected
+dotnet_naming_symbols.types.required_modifiers =
+
+dotnet_naming_symbols.non_field_members.applicable_kinds = property, event, method
+dotnet_naming_symbols.non_field_members.applicable_accessibilities = public, internal, private, protected, protected_internal, private_protected
+dotnet_naming_symbols.non_field_members.required_modifiers =
+
+# Naming styles
+
+dotnet_naming_style.begins_with_i.required_prefix = I
+dotnet_naming_style.begins_with_i.required_suffix =
+dotnet_naming_style.begins_with_i.word_separator =
+dotnet_naming_style.begins_with_i.capitalization = pascal_case
+
+dotnet_naming_style.pascal_case.required_prefix =
+dotnet_naming_style.pascal_case.required_suffix =
+dotnet_naming_style.pascal_case.word_separator =
+dotnet_naming_style.pascal_case.capitalization = pascal_case
+
+dotnet_naming_style.pascal_case.required_prefix =
+dotnet_naming_style.pascal_case.required_suffix =
+dotnet_naming_style.pascal_case.word_separator =
+dotnet_naming_style.pascal_case.capitalization = pascal_case
+dotnet_style_operator_placement_when_wrapping = beginning_of_line
+dotnet_style_coalesce_expression = true:suggestion
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+dotnet_style_prefer_is_null_check_over_reference_equality_method = true:suggestion
+dotnet_style_prefer_auto_properties = true:silent
+dotnet_style_object_initializer = true:suggestion
+dotnet_style_collection_initializer = true:suggestion
+dotnet_style_prefer_simplified_boolean_expressions = true:suggestion
+dotnet_style_prefer_conditional_expression_over_assignment = true:silent
+dotnet_style_prefer_conditional_expression_over_return = true:silent
+dotnet_style_explicit_tuple_names = true:suggestion
+dotnet_style_prefer_inferred_tuple_names = true:suggestion
+
+csharp_indent_labels = one_less_than_current
+csharp_using_directive_placement = outside_namespace:silent
+csharp_prefer_simple_using_statement = true:suggestion
+csharp_prefer_braces = true:silent
+csharp_style_namespace_declarations = file_scoped:warning
+csharp_style_prefer_method_group_conversion = true:silent
+csharp_style_prefer_top_level_statements = true:silent
+csharp_style_prefer_primary_constructors = true:suggestion
+csharp_style_expression_bodied_methods = false:silent
+csharp_style_expression_bodied_constructors = false:silent
+csharp_style_expression_bodied_operators = false:silent
+csharp_style_expression_bodied_properties = true:silent
+csharp_style_expression_bodied_indexers = true:silent
+csharp_style_expression_bodied_accessors = true:silent
+csharp_style_expression_bodied_lambdas = true:silent
+csharp_style_expression_bodied_local_functions = false:silent