Re-review after main branch update: all JetStream issues still present

The original PR was split and merged as separate commits (nats-io#734, nats-io#822,
nats-io#823, nats-io#824, nats-io#644). Server pool changes were dropped. Re-checked all 40
issues against current main (a9bfe71): 0 of 33 in-scope issues were
addressed. Added review of new commit nats-io#644 (watcher sentinel fix).

https://claude.ai/code/session_014UCS5Nmn5ZM9RoAxEKd3mA

Author: claude