Use Exit terminal values for short-circuit in flatMap callbacks

[EtaCassiopeia]

Summary

ZIO.succeed(x), ZIO.unit, and ZIO.none create a Sync node that the ZIO runloop must interpret. Exit values (Exit.succeed(x), Exit.unit, Exit.none) are pre-allocated terminal values the runloop can short-circuit on, avoiding the interpretation step.

Applied in five hot-path callbacks where the value is unconditionally terminal:

• FeatureFlagsLive.checkProviderStatus — ZIO.unit → Exit.unit (catch-all after error cases)
• FeatureFlagsLive.currentEvaluatedFlags — ZIO.succeed(Map.empty) → Exit.succeed(Map.empty) (no-transaction branch)
• Hook.scala structuredLogging.before — ZIO.unit → Exit.unit (level-disabled branch)
• FeatureFlagRegistry.getClient — ZIO.succeed(c) → Exit.succeed(c) (cache-hit branch)
• FeatureFlagRegistry.defaultClient — same

Credit

Spotted in #110 by @guizmaii.

Test plan

• All 340 tests pass
• scalafmtCheckAll clean

[EtaCassiopeia]

Tested independently; closing in favor of #110.