[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/FeatureControlFlag.swift

@@ -0,0 +1,61 @@
+import Foundation
+
+/// One evaluated flag, as returned inside `EvaluateResponse.result.flags[key]`.
+public struct FeatureControlFlag: Decodable, Sendable, Equatable {
+    public let enabled: Bool
+    public let variant: String?
+    public let payload: FeatureControlJSON?
+    public let reason: String?
+
+    public init(enabled: Bool, variant: String?, payload: FeatureControlJSON?, reason: String?) {
+        self.enabled = enabled
+        self.variant = variant
+        self.payload = payload
+        self.reason = reason
+    }
+}
+
+/// `EvaluateResponse.result` — the batch evaluation result for a set of keys.
+public struct FeatureControlSnapshot: Sendable, Equatable {
+    public let evaluatedAt: Date
+    public let configVersion: Int
+    public let flags: [String: FeatureControlFlag]
+
+    public init(evaluatedAt: Date, configVersion: Int, flags: [String: FeatureControlFlag]) {
+        self.evaluatedAt = evaluatedAt
+        self.configVersion = configVersion
+        self.flags = flags
+    }
+}
+
+extension FeatureControlSnapshot: Decodable {
+    private enum CodingKeys: String, CodingKey {
+        case evaluatedAt, configVersion, flags
+    }
+
+    public init(from decoder: Decoder) throws {
+        let container = try decoder.container(keyedBy: CodingKeys.self)
+        let rawDate = try container.decode(String.self, forKey: .evaluatedAt)
+        guard let date = FeatureControlDateParsing.parse(rawDate) else {
+            throw DecodingError.dataCorruptedError(forKey: .evaluatedAt, in: container,
+                                                    debugDescription: "Unrecognized date format: \(rawDate)")
+        }
+        self.evaluatedAt = date
+        self.configVersion = try container.decode(Int.self, forKey: .configVersion)
+        self.flags = try container.decode([String: FeatureControlFlag].self, forKey: .flags)
+    }
+}
+
+/// Wire envelope for `POST /api/feature-control/evaluate` — `{error, code, result}`.
+struct FeatureControlEvaluateEnvelope: Decodable, Sendable {
+    let error: Bool
+    let code: String?
+    let result: FeatureControlSnapshot
+}
+
+/// Request body for `POST /api/feature-control/evaluate`.
+struct FeatureControlEvaluateRequestBody: Encodable, Sendable {
+    let environment: String
+    let keys: [String]
+    let context: FeatureControlContext
+}