Feat: Add optimistic concurrency semantics to governance storage

[rian-be]

Summary

Add optimistic concurrency semantics to governance request storage so lifecycle transitions no longer depend on blind saves.

Added

• MutationRequest.Revision as the governance concurrency guard
• IMutationRequestStore.Create(...) and TryStore(..., expectedRevision) for explicit create and guarded update paths
• MutationRequestAlreadyExistsException for duplicate request creation attempts
• MutationRequestConcurrencyException for lifecycle transition races
• runtime helpers for lifecycle transitions and version resolution composition
• regression coverage for stale lifecycle snapshots and guarded store behavior

Changed

• governance lifecycle transitions now persist through compare and swap semantics instead of non atomic update flow
• in-memory governance storage now rejects duplicate creates and stale revision writes
• governance runtime orchestration is split into smaller helpers for transition validation, execution, and version-resolution result building
• version resolution persistence characterization uses the new create path

Result

Governance request storage now exposes explicit create and optimistic concurrency update semantics. Concurrent lifecycle transitions no longer both succeed against the same stale snapshot, and the runtime reports a clear concurrency failure when a guarded transition loses the race.

Testing

• dotnet build src/ModularityKit.Mutator.Governance.csproj -c Release
• dotnet test Tests/ModularityKit.Mutator.Governance.Tests/ModularityKit.Mutator.Governance.Tests.csproj -c Release
• dotnet build ModularityKit.Mutator.slnx -c Release

Linked Issues

• Closes [Task]: Make governance lifecycle transitions atomic #10
• Closes [Task]: Add optimistic concurrency semantics to governance request storage #11

Checklist

• runtime contract uses explicit create and guarded update paths
• stale lifecycle transitions are rejected through optimistic concurrency
• governance tests cover the redesigned storage semantics