[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/FeatureControlJSONTests.swift

@@ -0,0 +1,40 @@
+import XCTest
+@testable import LCFeatureControl
+
+final class FeatureControlJSONTests: XCTestCase {
+
+    private func decode(_ json: String) throws -> FeatureControlJSON {
+        try JSONDecoder().decode(FeatureControlJSON.self, from: Data(json.utf8))
+    }
+
+    func testDecodesNull() throws {
+        XCTAssertEqual(try decode("null"), .null)
+    }
+
+    func testDecodesBool() throws {
+        XCTAssertEqual(try decode("true"), .bool(true))
+    }
+
+    func testDecodesNumber() throws {
+        XCTAssertEqual(try decode("5"), .number(5))
+    }
+
+    func testDecodesString() throws {
+        XCTAssertEqual(try decode("\"hello\""), .string("hello"))
+    }
+
+    func testDecodesArray() throws {
+        XCTAssertEqual(try decode("[\"a\",\"b\"]"), .array([.string("a"), .string("b")]))
+    }
+
+    func testDecodesNestedObject() throws {
+        let json = #"{"limit": 5, "tags": ["a","b"], "nested": {"x": true}}"#
+        let value = try decode(json)
+        guard case let .object(object) = value else {
+            return XCTFail("expected .object, got \(value)")
+        }
+        XCTAssertEqual(object["limit"], .number(5))
+        XCTAssertEqual(object["tags"], .array([.string("a"), .string("b")]))
+        XCTAssertEqual(object["nested"], .object(["x": .bool(true)]))
+    }
+}