Feat: Clarify governance revalidation semantics and enum docs

[rian-be]

Summary

This PR hardens governance version resolution so revalidation becomes an explicit runtime state instead of an implicit branch.

Added

• pending revalidation semantics through PendingMutationReason.Revalidation
• end to end coverage for stale approval renewal and re-execution
• versioned resolution example updates that show revalidation behavior
• XML docs for public governance enums

Changed

• revalidation now persists as visible pending request state instead of silently staying approved
• version aware resolution history now records revalidation decisions explicitly
• versioned resolution example documentation now describes the revalidation pending path
• public governance enums now have complete XML documentation

Result

Governance version resolution is now easier to audit and reason about, with revalidation represented as first class runtime state and covered by tests and examples.

Testing

• dotnet test Tests/ModularityKit.Mutator.Governance.Tests/ModularityKit.Mutator.Governance.Tests.csproj -c Release
• dotnet build Examples/Governance/VersionedResolution/VersionedResolution.csproj -c Release

Linked Issues

• Closes [Task]: Harden governance version resolution semantics #19

Checklist

• Revalidation has explicit runtime semantics
• Revalidation is represented as a visible pending request state
• Stale approval resolution is covered by tests
• Example coverage shows the revalidation branch
• Public governance enums have XML docs