Fix duplicate rotation triggered by Write() after manual Rotate()/RotateWithReason()

[Copilot]

Rotate() and RotateWithReason() never updated lastRotationTime, so the very next Write() would re-evaluate the interval/scheduled guards, find them still expired, and fire a second unwanted rotation.

Changes

• timberjack.go — after a successful l.rotate(r), set l.lastRotationTime = l.resolvedTimeNow() so Write()'s interval and scheduled-time guards see the rotation as already completed. Updated doc comment to reflect the new behaviour.
• timberjack_test.go — added two regression tests:
  • TestRotate_NoDuplicateRotationOnNextWrite
  • TestRotateWithReason_NoDuplicateRotationOnNextWrite

Example

l := &Logger{Filename: "app.log", RotationInterval: time.Hour}
l.Write([]byte("boot"))         // initialises lastRotationTime

// two hours later — interval is due
l.Rotate()                       // rotates → before fix, lastRotationTime NOT updated
l.Write([]byte("next entry"))   // before fix: triggers a SECOND rotation immediately
                                 // after fix:  writes normally, no extra rotation

[DeRuina]

LGTM

[DeRuina]

LGTM