[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/FeatureControlDateParsing.swift

@@ -0,0 +1,18 @@
+import Foundation
+
+/// Parses the ISO-8601 timestamps Atomenta sends (`2026-04-16T12:00:00.000Z`, with
+/// milliseconds). `LCEssentials.API`'s internal `JSONDecoder` uses the default
+/// (`.deferredToDate`, numeric epoch) strategy, so `Date` fields on wire models
+/// decode the raw string manually instead of relying on `Decodable`'s default
+/// date handling.
+enum FeatureControlDateParsing {
+    static func parse(_ string: String) -> Date? {
+        let withFractional = ISO8601DateFormatter()
+        withFractional.formatOptions = [.withInternetDateTime, .withFractionalSeconds]
+        if let date = withFractional.date(from: string) { return date }
+
+        let plain = ISO8601DateFormatter()
+        plain.formatOptions = [.withInternetDateTime]
+        return plain.date(from: string)
+    }
+}