fix(radiance): account for grace period in displayed countdown and alert threshold

[Good627]

Summary

Follow-up to #20: adjusts Radiance rotation tracker countdown timers and alert thresholds to cleanly account for the configured grace period advance notice.

Details

• Displays remaining countdown on active local Radiance as \selfDuration - grace\ (matching remote countdown behavior).
• Cast prompt alert (\CAST RADIANCE) triggers promptly when the grace period countdown reaches 0s (i.e. \grace\ seconds before the buff expires) rather than requiring 0s of raw duration remaining.
• Prevents false-positive alert trigger on the casting player during local cast.
• Unit test suite updated and extended with \ estGracePeriodAlertTiming\ in \ est/tel/eden/mod/war/RadianceTrackerTest.java.