core: Add wall clock duration to CPU usage logging

core: Add wall clock duration to CPU usage logging

Enhance CPU time logging to include wall clock duration alongside
CPU consumption. When a unit transitions to inactive/failed state,
the log message now shows both CPU time consumed and the total wall
clock time since activation.

Changes:
- Calculate wall clock duration using active_enter_timestamp
- Update log format: "Consumed Xs CPU time over Ys wall clock time"
- Fallback to original format if no activation timestamp available
- Use monotonic clock for accurate duration calculation

This addresses issue #35738 by providing administrators better context
about service performance and resource efficiency.

Example output:
- With wall clock: "service: Consumed 30s CPU time over 5min wall clock time"
- Without timestamp: "service: Consumed 30s CPU time"