Ensure cohort is never nil:
- NewDomainSet stores an empty Cohort before rebuildCohort, so a parse
failure leaves an empty (non-nil) cohort instead of nil
- Contains uses CompareAndSwap to lazily self-initialise a zero-value
struct, making nil impossible after first access — no explicit nil
check needed
Extract the cache-validity check into a testable cachedCohortValid()
helper and add filesPresent() to detect when data files exist on disk.
Use strings.Builder in commaify() for efficiency.
Tests:
- TestPrefixSetContainsZeroValue: zero-value self-heals via CAS,
cohort is non-nil after first Contains call
- TestCachedCohortValidRequiresFiles: cache invalidation logic
