[sub-spm-optional-products] Add LCECryptoKit and LCFeatureControl as optional sub-SPM products

LCEssentials installs standalone; each sub target-depends on it so linking a
sub's product always pulls LCEssentials in too, without the consumer having to
declare it separately.

- LCECryptoKit: internalized from the remote LCECryptoKitBinary git dependency
  (embedded token URL removed) into a local binaryTarget vendoring
  Frameworks/LCECryptoKit.xcframework. LCECryptoKitManager moved out of
  LCEssentials core into its own LCECryptoKitManager target/product; the
  no-op fallback for when the binary wasn't linked is gone (breaking change
  for existing consumers, see decisions/2026-09-15-sub-spm-optional-products.md).
- LCFeatureControl: new product wrapping Atomenta's Feature Control API
  (flag evaluation with TTL cache + safe-degrade fallback to defaults,
  notifications inbox, batched exposure telemetry). 45 new tests.

Sources/LCFeatureControl/FeatureControlExposureEvent.swift

@@ -0,0 +1,28 @@
+import Foundation
+
+/// One exposure event for `POST /api/feature-control/telemetry/exposure`.
+public struct FeatureControlExposureEvent: Encodable, Sendable, Equatable {
+    public let featureKey: String
+    public let variant: String?
+    public let subjectType: FeatureControlSubjectType
+    public let storeId: String?
+
+    public init(featureKey: String, variant: String?, subjectType: FeatureControlSubjectType, storeId: String?) {
+        self.featureKey = featureKey
+        self.variant = variant
+        self.subjectType = subjectType
+        self.storeId = storeId
+    }
+}
+
+/// Request body — server caps `events` at 100 (`maxItems: 100`); batching into that
+/// limit is the caller's (`FeatureControlManager`'s) job, not this type's.
+struct FeatureControlExposureBatchBody: Encodable, Sendable {
+    let events: [FeatureControlExposureEvent]
+}
+
+/// Wire envelope for the telemetry response — `{error, code, result: {count}}`.
+struct FeatureControlExposureBatchEnvelope: Decodable, Sendable {
+    let error: Bool
+    let code: String?
+}