Issue: Add support for Once sync primitive with execution‑level isolation

Issue: Add support for Once sync primitive with execution‑level isolation: a task in LegoFlow-SWE (Harbor dataset). Shuttle currently does not provide a Once synchronization primitive. Code that relies on one‑time initialization (e.g., lazy init, singletons, initialisation flags) cannot be tested…

The task

Shuttle currently does not provide a `Once` synchronization primitive. Code that relies on one‑time initialization (e.g., lazy init, singletons, initialisation flags) cannot be tested under Shuttle, and if a workaround is attempted, static `Once` values are not properly isolated across separate Shuttle executions –…

Part of Lego-X/LegoFlow-SWE.