Run cleanup code on the relaunch API

Editor assessment

Concise and correctly categorized: adding cleanup execution before the relaunch API triggers is user-facing behavior, so `feat(core)` and the resulting minor bump are appropriate. The subject is imperative, lowercase, and names both the mechanism (`cleanup code`) and the surface (`relaunch API`), making it filterable; at roughly 50 characters it's the ideal length. Slight ambiguity: `cleanup code` could mean user-provided handlers or internal teardown—one clarifying word (`handlers`) would help, and a body would settle which resources are cleaned and in what order relative to process exit. No reference footers, so PR context lives elsewhere. Good; a touch under-explained.