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.

tests/Marathon.Application.Tests/PlaceholderTest.cs

@@ -0,0 +1,8 @@
+// Phase 4 will add real tests to this project.
+namespace Marathon.Application.Tests;
+
+public sealed class PlaceholderTest
+{
+    [Fact]
+    public void Placeholder_AlwaysPasses() => Assert.True(true);
+}