fix: report unreachable domains instead of 'No meaningful changes'

Closes #10.

resolvedSnapshotAfterFallback replaces a failed lookup's snapshot with
the previous one, so alertDescriptor compared the old snapshot against
itself, found matching hashes, and the run reported 'No meaningful
changes' for a domain that was never actually reached. Nothing in the
UI or the monitoring log distinguished that from a genuine no-change.

MonitoringDomainResult now carries unreachableReason, set when the
fallback fires. It is Optional so already-persisted monitoring logs
still decode. The run summary reads 'Could not check — kept the previous
result' with the underlying error, and monitoringEvents emits a warning-
severity monitoringFailure so configured integrations hear about it
rather than seeing silence.

1 files changed, 7 insertions, 1 deletions

diff --git a/DomainDig/Models.swift b/DomainDig/Models.swift
index b86fc07..6989cba 100644
--- a/DomainDig/Models.swift
+++ b/DomainDig/Models.swift
@@ -1544,6 +1544,10 @@ struct MonitoringDomainResult: Codable, Identifiable, Equatable {
     let certificateWarningLevel: CertificateWarningLevel
     let resultSource: LookupResultSource
     let errorMessage: String?
+    /// Non-nil when the lookup failed and the previous snapshot was reused, so
+    /// this run compared the old data against itself and cannot claim the
+    /// domain is unchanged. Optional so already-persisted logs still decode.
+    let unreachableReason: String?
 
     init(
         id: UUID = UUID(),
@@ -1555,7 +1559,8 @@ struct MonitoringDomainResult: Codable, Identifiable, Equatable {
         alertSeverity: MonitoringAlertSeverity?,
         certificateWarningLevel: CertificateWarningLevel,
         resultSource: LookupResultSource,
-        errorMessage: String? = nil
+        errorMessage: String? = nil,
+        unreachableReason: String? = nil
     ) {
         self.id = id
         self.domain = domain
@@ -1567,6 +1572,7 @@ struct MonitoringDomainResult: Codable, Identifiable, Equatable {
         self.certificateWarningLevel = certificateWarningLevel
         self.resultSource = resultSource
         self.errorMessage = errorMessage
+        self.unreachableReason = unreachableReason
     }
 }