test(installer): roll back persisted state on start failure

[vernonstinebaker]

Summary

• cover the installer path where instance state is written successfully but process start fails immediately afterward
• factor the persist-and-start handoff into a small helper so the rollback behavior can be tested directly
• remove the persisted instance entry and resave state when startup fails so failed installs do not appear installed

Validation

• zig build test -Dembed-ui=false -Dbuild-ui=false --summary all
• npm --prefix ui ci --no-audit --no-fund
• npm --prefix ui run build
• bash tests/test_e2e.sh
• zig fmt --check src/
• git diff --check

Notes

• this keeps the production install flow unchanged apart from failure cleanup after the state file has already been updated
• the new regression is intentionally targeted at the persisted-state boundary rather than broader installer setup so it stays deterministic and fast