fix: preserve fractional activity level calculation

[vycdev2]

Summary

• Preserve the fractional part of the activity-level formula instead of truncating the ratio through integer division.
• Add regression boundaries around the affected level transition.

Verification

• dotnet restore — passed (existing NU1903 vulnerability warning).
• dotnet build --no-restore — passed (existing NU1903 vulnerability warning).
• dotnet test --no-restore --filter FullyQualifiedName~ActivityLevelServiceTests — passed (13 tests).
• dotnet test --no-restore --filter 'FullyQualifiedName!~NormalizeTimeUntilEventName' — passed (299 tests).
• dotnet test --no-restore — failed only because the environment is globalization-invariant and two pre-existing tests require tr-tr; all other 299 tests passed.
• git diff --check — passed.

Risk

• Low: this changes only the numeric precision used by CalculateLevel; the regression tests cover the corrected boundary.

This was generated by an AI agent (vycdev2). Please verify any changes before merging or applying.