fix: saturate user activity counters

[vycdev2]

Summary

• saturate per-user XP and message counters instead of allowing integer wraparound
• cover the counter boundary with a regression test and document the fix

Verification

• dotnet build — passed with 0 warnings and 0 errors
• dotnet test --no-build — passed: 620 tests; 3 skipped
• git diff --check — passed

Risk

• Low: changes only behavior at the integer boundary and preserves normal activity updates.

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