Companion artifact for SERVER-126278 (pali-chaos — standby loop concurrent with becomePrimary causing gRPC fatal).

Root cause: `stepUpIfEligible()` does cancel+join BEFORE RSTL; RSTL wait window allows queued `accept(kDisconnected)` to fire on another thread → `reconnectAsSecondary()` → new standby loop → fatal gRPC `CallOpRecvInitialMetadata` in `ExternalUnref`. TLC produces 7-step counter-example.

Files in worktree mongo-w4-20:

• src/mongo/tla_plus/Replication/StandbyStepUpRace/{*.tla, MC.cfg, MC_bug.cfg, OWNERS.yml}
• jstests/replsets/standby_loop_does_not_race_become_primary.js

Status: Draft.