fix(ledger): harden leader schedule cache

[wolf31o2]

Summary by cubic

Hardened the leader schedule cache to avoid races during start/stop and improved leader slot lookup near epoch boundaries to reduce false negatives.

• Bug Fixes
  • Made scheduleComputeLoop consume a passed computeCh and set e.computeCh to nil on Stop to prevent sends after shutdown.
  • Guarded compute/store paths to no-op when the election is not running.
  • Fixed NextLeaderSlot to handle missing schedules safely and check the next epoch’s cache when near boundaries.
  • Stabilized tests by using atomic currentEpoch and waiting for the initial schedule.
  • Added .worktrees to .gitignore.

^{Written for commit f5c9e8f. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 3 files