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.

plans/initial-implementation/PLAN.md

@@ -35,7 +35,7 @@ parameter configurable.
 ## Phases
 
 - [x] Phase 0: Scraping spike (research, throwaway) [domain: backend] → [subplan](./phase-0-scraping-spike.md)
-- [ ] Phase 1: Solution skeleton + Domain model [domain: backend] → [subplan](./phase-1-solution-and-domain.md)
+- [x] Phase 1: Solution skeleton + Domain model [domain: backend] → [subplan](./phase-1-solution-and-domain.md)
 - [ ] Phase 2: Infrastructure — Storage [domain: backend] → [subplan](./phase-2-storage.md)
 - [ ] Phase 3: Infrastructure — Scraping [domain: backend] → [subplan](./phase-3-scraping.md)
 - [ ] Phase 4: Application layer + Background workers [domain: backend] → [subplan](./phase-4-application-and-workers.md)
@@ -63,7 +63,7 @@ parameter configurable.
 | Phase | Domain | Status | Review | Build | Committed |
 |---|---|---|---|---|---|
 | Phase 0: Scraping spike | backend | ✅ Done | ⚠️ Pass with notes (Sonnet) | ⏭️ N/A (research) | ✅ 070e34b |
-| Phase 1: Solution + Domain | backend | ⬜ Not Started | ⬜ | ⬜ | ⬜ |
+| Phase 1: Solution + Domain | backend | ✅ Done | ⬜ | ✅ Build OK | ⬜ |
 | Phase 2: Storage | backend | ⬜ Not Started | ⬜ | ⏭️ Big Bang | ⬜ |
 | Phase 3: Scraping | backend | ⬜ Not Started | ⬜ | ⏭️ Big Bang | ⬜ |
 | Phase 4: Application + Workers | backend | ⬜ Not Started | ⬜ | ⏭️ Big Bang | ⬜ |