feat: implement Phase 1 — solution skeleton and domain model

Creates the 9-project .NET 8 solution (5 src + 4 test) with Marathon.Domain
fully implemented: value objects (SportCode, EventId, OddsRate, OddsValue,
BetScope hierarchy), enums (Side, BetType, OddsSource, AnomalyKind), and
entities (Sport, Country, League, Event, Bet, OddsSnapshot, EventResult,
Anomaly) with all invariants enforced in constructors. 96 domain tests pass
(FluentAssertions + xUnit). Directory.Build.props and Directory.Packages.props
centralise build settings and NuGet versions. Both Marathon.sln and Marathon.slnx
are committed; dotnet build Marathon.sln succeeds with 0 warnings/errors.

src/Marathon.UI/Component1.razor

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+<div class="my-component">
+    This component is defined in the <strong>Marathon.UI</strong> library.
+</div>

src/Marathon.UI/Component1.razor.css

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+.my-component {
+    border: 2px dashed red;
+    padding: 1em;
+    margin: 1em 0;
+    background-image: url('background.png');
+}

src/Marathon.UI/ExampleJsInterop.cs

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+using Microsoft.JSInterop;
+
+namespace Marathon.UI;
+
+// This class provides an example of how JavaScript functionality can be wrapped
+// in a .NET class for easy consumption. The associated JavaScript module is
+// loaded on demand when first needed.
+//
+// This class can be registered as scoped DI service and then injected into Blazor
+// components for use.
+
+public class ExampleJsInterop : IAsyncDisposable
+{
+    private readonly Lazy<Task<IJSObjectReference>> moduleTask;
+
+    public ExampleJsInterop(IJSRuntime jsRuntime)
+    {
+        moduleTask = new (() => jsRuntime.InvokeAsync<IJSObjectReference>(
+            "import", "./_content/Marathon.UI/exampleJsInterop.js").AsTask());
+    }
+
+    public async ValueTask<string> Prompt(string message)
+    {
+        var module = await moduleTask.Value;
+        return await module.InvokeAsync<string>("showPrompt", message);
+    }
+
+    public async ValueTask DisposeAsync()
+    {
+        if (moduleTask.IsValueCreated)
+        {
+            var module = await moduleTask.Value;
+            await module.DisposeAsync();
+        }
+    }
+}

src/Marathon.UI/Marathon.UI.csproj

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+<Project Sdk="Microsoft.NET.Sdk.Razor">
+
+  <PropertyGroup>
+    <TargetFramework>net8.0</TargetFramework>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <SupportedPlatform Include="browser" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.AspNetCore.Components.Web" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\Marathon.Domain\Marathon.Domain.csproj" />
+    <ProjectReference Include="..\Marathon.Application\Marathon.Application.csproj" />
+  </ItemGroup>
+
+</Project>

src/Marathon.UI/_Imports.razor

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+@using Microsoft.AspNetCore.Components.Web

src/Marathon.UI/wwwroot/exampleJsInterop.js

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+// This is a JavaScript module that is loaded on demand. It can export any number of
+// functions, and may import other JavaScript modules if required.
+
+export function showPrompt(message) {
+  return prompt(message, 'Type anything here');
+}