[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/FeatureControlCache.swift

@@ -0,0 +1,46 @@
+import Foundation
+
+/// Identifies a cached evaluation — FC-060 §5: "environment + subjectId + keys(sorted) +
+/// platform + appVersion". `sortedKeys` means caller `keys` order never affects hits.
+struct FeatureControlCacheKey: Hashable, Sendable {
+    let environment: String
+    let subjectId: String
+    let sortedKeys: [String]
+    let platform: String?
+    let appVersion: String?
+}
+
+/// In-memory-only TTL cache (no disk persistence — see SDD §2 Non-Goals). The clock is
+/// injectable so tests can force expiry deterministically instead of sleeping.
+actor FeatureControlCache {
+    private struct Entry {
+        let snapshot: FeatureControlSnapshot
+        let storedAt: Date
+        let ttl: TimeInterval
+    }
+
+    private var entries: [FeatureControlCacheKey: Entry] = [:]
+    private let now: @Sendable () -> Date
+
+    init(now: @escaping @Sendable () -> Date = { Date() }) {
+        self.now = now
+    }
+
+    /// `allowStale: true` returns an expired entry rather than `nil` — used by the
+    /// safe-degrade fallback path (SDD §4.4/§4.5).
+    func get(_ key: FeatureControlCacheKey, allowStale: Bool) -> FeatureControlSnapshot? {
+        guard let entry = entries[key] else { return nil }
+        let age = now().timeIntervalSince(entry.storedAt)
+        if age <= entry.ttl { return entry.snapshot }
+        return allowStale ? entry.snapshot : nil
+    }
+
+    /// Always overwrites — a fresh fetch's `configVersion` simply replaces whatever was there.
+    func set(_ key: FeatureControlCacheKey, snapshot: FeatureControlSnapshot, ttl: TimeInterval) {
+        entries[key] = Entry(snapshot: snapshot, storedAt: now(), ttl: ttl)
+    }
+
+    func invalidateAll() {
+        entries.removeAll()
+    }
+}