fix: RetryPolicy.withRetry closure label, missing Foundation import

RetryPolicy.withRetry's operation param wasn't anonymous (_), so the
14 Outpost call sites that pass the closure in parens - RetryPolicy.
withRetry({ ... }) - rather than as a trailing closure failed to
compile ("Missing argument label 'operation:'" cascading into
nonsense errors about maxAttempts). OutlineKit's own internal call
sites all happened to use trailing-closure syntax, so this only
showed up once the app target actually got compiled. One-line fix at
the declaration (_ operation:) instead of touching every call site -
trailing-closure calls are unaffected either way.

APIFailureCenter.swift used Date/TimeInterval/URL/URLComponents/
URLQueryItem while only importing Observation and OutlineKit -
missing import Foundation. Other new files in the same commit escaped
this because they import SwiftUI, which re-exports Foundation
transitively; this one didn't.

OutlineKit: 92/92 still passing.

OutlineKit/Sources/OutlineKit/Support/RetryPolicy.swift

@@ -14,7 +14,7 @@ public enum RetryPolicy {
     public static func withRetry<T: Sendable>(
         maxAttempts: Int = 3,
         initialDelay: Duration = .seconds(1),
-        operation: () async throws -> T
+        _ operation: () async throws -> T
     ) async throws -> T {
         var attempt = 1
         var delay = initialDelay

Outpost/Root/APIFailureCenter.swift

@@ -1,3 +1,4 @@
+import Foundation
 import Observation
 import OutlineKit