fix: harden refresh generation guards (#391)

This follow-up fixes the post-merge correctness gap left after #386.

- make refresh notification emission generation-aware and atomic with
the generation check
- keep stale refresh state from syncing back into shared locators after
a newer configure
- replace the missing-env one-shot boolean with generation-stamped
reservation semantics so newer generations can supersede stale
reservations
- add regression tests for stale-generation reporting and missing-env
reservation behavior

Fixes #390

Author: karthiknadig