[lcfeaturecontrol-review-fixes] Fix JWT log leak, drop macOS/tvOS, coalesce requests

- FeatureControlManager/NotificationsClient: pass debug: false at every
  api.request(...) call site — the API default (debug: true) was printing
  Authorization: Bearer <jwt> on every request, release builds included.
- Package.swift: drop macOS/tvOS from platforms — LCEssentials.API (used by
  LCFeatureControl) is iOS/watchOS-only and the vendored xcframework has no
  macOS/tvOS slice.
- FeatureControlManager: coalesce concurrent evaluate() calls on a cold cache
  into a single in-flight request per key instead of firing one per caller.
- FeatureControlManager/NotificationsClient: treat a 200 response carrying
  {"error": true} as a failure instead of caching/returning it as success.
- FeatureControlNotificationsClient: thread a cursor param through list() so
  the already-decoded nextCursor can actually be used to page.
- Tests: 9 new tests (coalescing, envelope-error-on-200, cursor param, a real
  object payload decoded through a full evaluate response, date-decode
  failures) and removed the 7 remaining force-unwraps in test scaffolding.
- Documentation/FeatureControl.md: new guide for the LCFeatureControl product,
  cross-linked from README.md and Extensions.md.

Tests/LCFeatureControlTests/FeatureControlErrorTests.swift

@@ -4,18 +4,10 @@ import XCTest
 
 final class FeatureControlErrorTests: XCTestCase {
 
-    private var api: API!
-
-    override func setUp() {
-        super.setUp()
-        let cfg = URLSessionConfiguration.ephemeral
-        cfg.protocolClasses = [StubURLProtocol.self]
-        api = API(testConfiguration: cfg)
-    }
+    private var api = API.lce_featureControlTestInstance()
 
     override func tearDown() {
         StubURLProtocol.reset()
-        api = nil
         super.tearDown()
     }