fix(triggers): retain rules when the legacy projection is missing

Sources/DilatedCore/TriggerStore.swift

@@ -32,6 +32,7 @@ public struct TriggerStore: Sendable {
       try persist(record.rules, previous: legacy)
       return record.rules
     }
+    guard FileManager.default.fileExists(atPath: legacyFile.path) else { return record.rules }
     let baseline = Dictionary(uniqueKeysWithValues: record.legacy.map { ($0.id, $0) })
     let current = Dictionary(uniqueKeysWithValues: legacy.map { ($0.id, $0) })
     var rules = record.rules.filter { baseline[$0.id] == nil || current[$0.id] != nil }

Tests/DilatedCoreTests/TriggerStoreTests.swift

@@ -0,0 +1,28 @@
+import Foundation
+import Testing
+
+@testable import DilatedCore
+
+struct TriggerStoreTests {
+  @Test(arguments: [false, true])
+  func missingProjectionPreservesRulesWhileAnEmptyProjectionDeletesThem(missing: Bool) throws {
+    let directory = FileManager.default.temporaryDirectory.appendingPathComponent(UUID().uuidString)
+    defer { try? FileManager.default.removeItem(at: directory) }
+    let store = TriggerStore(directory: directory)
+    let rule = TriggerRule(name: "AC", criteria: [.power(onBattery: false)])
+    try store.save([rule])
+    let projection = directory.appendingPathComponent("triggers.json")
+    if missing {
+      try FileManager.default.removeItem(at: projection)
+    } else {
+      try JSONEncoder().encode([TriggerRule]()).write(to: projection, options: .atomic)
+    }
+    let expected = missing ? [rule] : []
+    let loaded = try store.load()
+    #expect(loaded == expected)
+    try store.save(loaded)
+    #expect(try store.load() == expected)
+    #expect(
+      try JSONDecoder().decode([TriggerRule].self, from: Data(contentsOf: projection)) == expected)
+  }
+}