Why: Since the system-managed settings change (#188), runtime.* writes
at the user scope are rejected, so the mock startup flag failed with
"system-managed settings require admin" and the serve process never
became healthy - the Dev multi-instance integration regression.
Evidence: go build ok; internal/app and internal/settings suites green
(SELF); the Dev composite CI on the follow-up PR re-runs
make test-multi-instance against this fix (GATE).
Impact: The mock startup flag now seeds runtime.mock via MergeSystem
(trusted operator input, effective for every tenant through the
Effective merge chain); no HTTP surface changes.
Refs: Dev PR #508; backend #188.
weichen