[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.

Tests/LCFeatureControlTests/FeatureControlCacheTests.swift

@@ -0,0 +1,70 @@
+import XCTest
+@testable import LCFeatureControl
+
+/// Test-only mutable clock box — a `var` captured directly by the cache's
+/// escaping `@Sendable` closure would trip strict-concurrency capture checks;
+/// this mirrors the `@unchecked Sendable` pattern `StubURLProtocol` already uses.
+private final class MutableClock: @unchecked Sendable {
+    var value: Date
+    init(_ value: Date) { self.value = value }
+}
+
+final class FeatureControlCacheTests: XCTestCase {
+
+    private func snapshot(configVersion: Int) -> FeatureControlSnapshot {
+        FeatureControlSnapshot(evaluatedAt: Date(), configVersion: configVersion, flags: [:])
+    }
+
+    private func key(_ keys: [String] = ["a", "b"]) -> FeatureControlCacheKey {
+        FeatureControlCacheKey(environment: "production", subjectId: "cust_1",
+                               sortedKeys: keys.sorted(), platform: "ios", appVersion: "1.0")
+    }
+
+    func testHitWithinTTLReturnsSameSnapshot() async {
+        let cache = FeatureControlCache(now: { Date() })
+        await cache.set(key(), snapshot: snapshot(configVersion: 1), ttl: 45)
+        let hit = await cache.get(key(), allowStale: false)
+        XCTAssertEqual(hit?.configVersion, 1)
+    }
+
+    func testMissAfterTTLExpiryReturnsNilWhenStaleNotAllowed() async {
+        let clock = MutableClock(Date())
+        let cache = FeatureControlCache(now: { clock.value })
+        await cache.set(key(), snapshot: snapshot(configVersion: 1), ttl: 1)
+        clock.value = clock.value.addingTimeInterval(2)
+        let hit = await cache.get(key(), allowStale: false)
+        XCTAssertNil(hit)
+    }
+
+    func testStaleAllowedReturnsExpiredEntry() async {
+        let clock = MutableClock(Date())
+        let cache = FeatureControlCache(now: { clock.value })
+        await cache.set(key(), snapshot: snapshot(configVersion: 1), ttl: 1)
+        clock.value = clock.value.addingTimeInterval(2)
+        let hit = await cache.get(key(), allowStale: true)
+        XCTAssertEqual(hit?.configVersion, 1)
+    }
+
+    func testKeyDiffersBySortedKeysNotInputOrder() async {
+        let cache = FeatureControlCache(now: { Date() })
+        await cache.set(key(["a", "b"]), snapshot: snapshot(configVersion: 1), ttl: 45)
+        let hit = await cache.get(key(["b", "a"]), allowStale: false)
+        XCTAssertEqual(hit?.configVersion, 1)
+    }
+
+    func testSetOverwritesPriorConfigVersion() async {
+        let cache = FeatureControlCache(now: { Date() })
+        await cache.set(key(), snapshot: snapshot(configVersion: 7), ttl: 45)
+        await cache.set(key(), snapshot: snapshot(configVersion: 8), ttl: 45)
+        let hit = await cache.get(key(), allowStale: false)
+        XCTAssertEqual(hit?.configVersion, 8)
+    }
+
+    func testInvalidateAllClearsEntries() async {
+        let cache = FeatureControlCache(now: { Date() })
+        await cache.set(key(), snapshot: snapshot(configVersion: 1), ttl: 45)
+        await cache.invalidateAll()
+        let hit = await cache.get(key(), allowStale: true)
+        XCTAssertNil(hit)
+    }
+}