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.

CLAUDE.md

@@ -102,7 +102,13 @@ Marathon_<YYYY-MM-DD>_to_<YYYY-MM-DD>.xlsx
 
 ## Recurring Issues & Patterns
 
-(Populated as we work — leave empty until something repeats.)
+- **`dotnet new sln` on .NET 10 SDK produces `.slnx`**, not `.sln`. If the plan
+  references `Marathon.sln`, hand-craft the traditional format alongside `.slnx`.
+- **`Marathon.Application` namespace vs `System.Windows.Application`:** in any WPF
+  project that references `Marathon.Application`, always write
+  `System.Windows.Application` fully qualified in `App.xaml.cs`.
+- **`Directory.Build.props` must NOT set `TargetFramework`** when projects in the
+  same solution use different TFMs (e.g., `net8.0` vs `net8.0-windows`).
 
 ## Feature: Initial Implementation > Phase 0: Scraping Spike — Learnings