[mattmacrocket]

Closes #63.

Why

An attacker who finds the sensor host has stopping it as the obvious first move. Today the app notices after five minutes, appends a Medium alert to its own list, and tells nobody. Two gaps, both fixed here:

1. The synthetic alert never went through onAlertNotificationsRequested, so it did not reach Notification Center at all. With the window closed, nothing showed.
2. Nothing left the Mac. The sensor is the only Slack sender, and the sensor is the thing that is gone.

What

Connection service (SensorConnectionService)

• After the outage delay the app raises system.sensor_offline, adds it to the feed, routes it through the same Notification Center hook WebSocket alerts use, then hands it to an injected offlineDelivery closure.
• Severity: High when the sensor had been .live earlier in this app session, Medium when it never connected.
• Guard fix: the old timer required exactly .disconnected. Reconnects run every 30 s, so a health check in flight at the five-minute mark silently dropped the alert for the rest of the outage. isOffline now treats .connecting as offline; authFailed and incompatible are not, since the sensor answered.
• offlineAlertDelay is injectable (default 300 s) so the timer is testable.

Out-of-band delivery (Desktop/OfflineAlertDelivery.swift, new)

• SlackAlertFormatter mirrors the sensor's format_slack_payload block for block (header, severity/type fields, time or source-IP section, italic detail, same emoji map), so an app-sent message is indistinguishable in the channel.
• SlackDeliveryCache: the sensor redacts webhook_url on every read, so the only moment the app holds the real URL is when the operator types it in Settings. The cache stores it then, as a Keychain generic password under the app's service name; enabled and minimum severity live in defaults and refresh from every config load. The redaction sentinel never overwrites a cached URL; an emptied field deletes it; only https URLs with a host are accepted.
• OfflineAlertDispatcher: honors enabled and minimum severity, POSTs through an ephemeral URLSession, never throws (the caller is a connection-loss path), and reports delivered / skipped(reason) / failed(message).
• AlertSeverityRank is shared with MacNotificationPolicy instead of a second private copy.

Settings: saveAlertMethod("slack", …) and the Slack branch of loadConfig both feed the cache.

App: wires OfflineAlertDispatcher.shared into the connection service.

Scoping decision

APNs stays sensor-side. The app never receives relay_url/relay_token/device_token (redacted) and has no UI to enter them, so there is nothing to cache. The docs say so explicitly.

Security model

One new place a configuration credential lives: the Slack webhook URL in the Mac app's Keychain, written only on operator entry, deleted on clear, used only for this alert, posted over an ephemeral session. docs/SECURITY_MODEL.md "Stored secrets" and docs/USER_GUIDE.md "Credential Safety" record it. Control surface only; nothing attacker-visible changes.

Tests

Red first (compile failures on the new types and parameters), then green:

• SlackAlertFormatterTests: block shape matches the sensor for an offline alert; emoji map; source-IP section variant.
• SlackDeliveryCacheTests: real URL cached; redaction sentinel ignored; disable keeps URL; empty clears; non-https rejected; empty cache.
• OfflineAlertDispatcherTests: posts formatted JSON to the cached webhook; below-minimum skipped; disabled and unconfigured skipped; HTTP 500 reported not thrown.
• OfflineAlertOutOfBandTests (connection service, 50 ms delay): never-connected outage is Medium and reaches both Notification Center and the delivery hook; losing a live sensor is High; isOffline table; synthetic severity.
• MockWSManager gained finishStreamAfterFrames so a test can take the service from live to disconnected.

Full swift test: 421 + 121 + 39 passed.

Docs

USER_GUIDE.md: Sensor Disconnected row (Medium or High, delivered by the app), the reconnect paragraph, a "Where deliveries come from" note under Alert Methods, Credential Safety. SECURITY_MODEL.md: Stored secrets.