release: bump version to 0.12.0 — Database & Repository milestone com…

…plete

- Update VersionPrefix from 0.11.0 to 0.12.0 in Directory.Build.props
- Mark CHANGELOG [Unreleased] as [0.12.0] - 2026-02-16
- Update ROADMAP.md: v0.12.0 marked as COMPLETED (58 issues)
- Update INVENTORY.md: v0.12.0 milestone marked as completed
- Update docs/releases/v0.12.0/README.md: status → Completed
- Close GitHub milestone v0.12.0 - Database & Repository
- Add generate-issues-excel.cs script for issue history export

feat(testing): add Encina.Testing.Fakes package

Implements Issue #426 - Test doubles for IEncina and messaging stores.

New package: Encina.Testing.Fakes
- FakeEncina: Thread-safe IEncina implementation with verification
  - SetupResponse/SetupError/SetupStream for configuration
  - WasSent/WasPublished/GetSentRequests for verification
  - Order-preserving collections using List<T> with locking
- FakeOutboxStore, FakeInboxStore, FakeSagaStore
- FakeScheduledMessageStore, FakeDeadLetterStore
- ServiceCollectionExtensions for DI registration

Tests: 42 unit tests covering all functionality

Fixes #426

fix: move IAggregate and AggregateBase to DomainModeling

Fixes #494

Move event sourcing base types from Encina.Marten to
Encina.DomainModeling to eliminate the transitive dependency on
DistributedLock.Core 1.0.8, which lacks .NET 10 support.

Changes:
- Create IAggregate and AggregateBase in Encina.DomainModeling
- Update Encina.Testing to depend on DomainModeling instead of Marten
- Add DomainModeling reference to Encina.Marten
- Update all namespace imports across affected files
- Re-enable Marten and Testing tests in CI workflows