feat(v3.2.0): add Data+ tier and external data integrations

- introduce Data+ tier for advanced data features
- add ownership history and DNS history
- expand subdomain discovery with external sources
- add domain pricing insights
- implement local usage/credit system
- integrate external data service layer with rate limiting
- extend export and CLI for Data+ features

1 files changed, 73 insertions, 0 deletions

diff --git a/DomainDig/UsageCreditService.swift b/DomainDig/UsageCreditService.swift
new file mode 100644
index 0000000..48f40be
--- /dev/null
+++ b/DomainDig/UsageCreditService.swift
@@ -0,0 +1,73 @@
+import Foundation
+
+actor UsageCreditService {
+    static let shared = UsageCreditService()
+
+    private struct CreditLedger: Codable {
+        let appVersion: String
+        var remainingByFeature: [UsageCreditFeature: Int]
+    }
+
+    private let storageKey = "usageCredits.ledger"
+    private var ledger: CreditLedger
+
+    init(defaults: UserDefaults = .standard) {
+        if let data = defaults.data(forKey: storageKey),
+           let decoded = try? JSONDecoder().decode(CreditLedger.self, from: data),
+           decoded.appVersion == AppVersion.current {
+            ledger = decoded
+        } else {
+            ledger = Self.makeLedger()
+            if let data = try? JSONEncoder().encode(ledger) {
+                defaults.set(data, forKey: storageKey)
+            }
+        }
+    }
+
+    func status(for feature: UsageCreditFeature) -> UsageCreditStatus {
+        let total = feature.defaultAllowance
+        let remaining = ledger.remainingByFeature[feature] ?? total
+        return UsageCreditStatus(
+            feature: feature,
+            remaining: remaining,
+            total: total,
+            resetContext: "Resets with app version \(ledger.appVersion)"
+        )
+    }
+
+    func allStatuses() -> [UsageCreditStatus] {
+        UsageCreditFeature.allCases.map { status(for: $0) }
+    }
+
+    func canUse(_ feature: UsageCreditFeature) -> Bool {
+        status(for: feature).remaining > 0
+    }
+
+    @discardableResult
+    func consume(_ feature: UsageCreditFeature) -> UsageCreditStatus {
+        let current = ledger.remainingByFeature[feature] ?? feature.defaultAllowance
+        ledger.remainingByFeature[feature] = max(0, current - 1)
+        persist()
+        return status(for: feature)
+    }
+
+    func resetForCurrentVersion() {
+        ledger = Self.makeLedger()
+        persist()
+    }
+
+    private static func makeLedger() -> CreditLedger {
+        CreditLedger(
+            appVersion: AppVersion.current,
+            remainingByFeature: Dictionary(
+                uniqueKeysWithValues: UsageCreditFeature.allCases.map { ($0, $0.defaultAllowance) }
+            )
+        )
+    }
+
+    private func persist(defaults: UserDefaults = .standard) {
+        if let data = try? JSONEncoder().encode(ledger) {
+            defaults.set(data, forKey: storageKey)
+        }
+    }
+}