Run shutdown Ref.sets sequentially

[EtaCassiopeia]

Summary

FeatureFlags#shutdown was running five Ref.set calls via ZIO.collectAllParDiscard. Atomic Ref.set is a lock-free nanosecond-scale operation, so the fork/join overhead of parallel execution exceeds the work itself. A sequential *> chain is clearer and avoids fiber creation.

// Before
ZIO.collectAllParDiscard(List(
  state.statusRef.set(ProviderStatus.NotReady),
  state.hooksRef.set(List.empty),
  state.globalContextRef.set(EvaluationContext.empty),
  state.clientContextRef.set(EvaluationContext.empty),
  state.trackRecorder.set(List.empty)
)) *> state.eventHub.shutdown *> ZIO.attemptBlocking(api.shutdown()).ignore

// After
state.statusRef.set(ProviderStatus.NotReady) *>
  state.hooksRef.set(List.empty) *>
  state.globalContextRef.set(EvaluationContext.empty) *>
  state.clientContextRef.set(EvaluationContext.empty) *>
  state.trackRecorder.set(List.empty) *>
  state.eventHub.shutdown *>
  ZIO.attemptBlocking(api.shutdown()).ignore

Pure refactor; no behavior change (these Refs are independent so any order is correct).

Credit

Spotted in #110 by @guizmaii.

Test plan

• All tests pass (340 + 99 + 170)
• scalafmtCheckAll clean

[EtaCassiopeia]

Tested independently; closing in favor of #110.