[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/Support/StubURLProtocol.swift

@@ -1,4 +1,15 @@
 import Foundation
+@testable import LCEssentials
+
+/// Shared factory so test classes don't each declare a force-unwrapped `API!`
+/// var for the XCTest setUp/tearDown lifecycle.
+extension API {
+    static func lce_featureControlTestInstance() -> API {
+        let cfg = URLSessionConfiguration.ephemeral
+        cfg.protocolClasses = [StubURLProtocol.self]
+        return API(testConfiguration: cfg)
+    }
+}
 
 /// Test double for `URLProtocol`. Intercepts every request on a `URLSession`
 /// configured with it, records the outgoing `URLRequest`, and replays a canned
@@ -56,6 +67,13 @@ final class StubURLProtocol: URLProtocol, @unchecked Sendable {
         return _capturedRequests.count
     }
 
+    /// Bodies of every intercepted request, in order — for tests asserting batch
+    /// boundaries across multiple requests (`lastCapturedBody` only sees the last one).
+    static var capturedBodies: [Data] {
+        lock.lock(); defer { lock.unlock() }
+        return _capturedBodies
+    }
+
     private static func currentStub() -> Stub {
         lock.lock(); defer { lock.unlock() }
         return _stub
@@ -84,11 +102,14 @@ final class StubURLProtocol: URLProtocol, @unchecked Sendable {
             return
         }
 
-        let url = request.url ?? URL(string: "https://stub.invalid")!
-        let response = HTTPURLResponse(url: url,
-                                       statusCode: stub.statusCode,
-                                       httpVersion: "HTTP/1.1",
-                                       headerFields: stub.headers)!
+        guard let url = request.url ?? URL(string: "https://stub.invalid"),
+              let response = HTTPURLResponse(url: url,
+                                             statusCode: stub.statusCode,
+                                             httpVersion: "HTTP/1.1",
+                                             headerFields: stub.headers) else {
+            client.urlProtocol(self, didFailWithError: URLError(.badServerResponse))
+            return
+        }
         client.urlProtocol(self, didReceive: response, cacheStoragePolicy: .notAllowed)
         client.urlProtocol(self, didLoad: stub.body)
         client.urlProtocolDidFinishLoading(self)