fix: fix thread-safety crash in Matcher when tests run in parallel

The `Matcher` singleton's `matchers` array is accessed without
synchronization. Under Swift Testing's parallel execution, concurrent
`register()` writes and `comparator()` reads from multiple test
`init()` methods cause data races that crash the test process.

Observed crash sites:
- `Matcher.register<A>(_:)` — concurrent appends to the array
- `closure #1 in Parameter.eraseToGenericValue()` — concurrent reads
  via `Matcher.comparator(for:)` while another thread appends

Add an NSLock to synchronize all matchers array access. The lock is
applied at the leaf level (append and snapshot-read) to avoid
re-entrancy in the Sequence comparator path which calls
`comparator(by:)` → `comparator(for: T.Element.self)` →
`comparator(by:)`.

Author: fortmarek

Sources/Mockable/Matcher/Matcher.swift

@@ -34,7 +34,7 @@ public class Matcher {
 
     // MARK: Private Properties
 
-    private var matchers: [MatcherType] = []
+    private let matchers = LockedValue<[MatcherType]>([])
 
     #if swift(>=6)
     nonisolated(unsafe) private static var `default` = Matcher()
@@ -133,7 +133,7 @@ public class Matcher {
 extension Matcher {
     private func register<T>(_ valueType: T.Type, match: @escaping Comparator<T>) {
         let mirror = Mirror(reflecting: valueType)
-        matchers.append((mirror, match as Any))
+        matchers.withValue { $0.append((mirror, match as Any)) }
     }
 
     private func register<T>(_ valueType: T.Type.Type) {
@@ -143,7 +143,7 @@ extension Matcher {
     private func register<T>(_ valueType: T.Type) where T: Equatable {
         let mirror = Mirror(reflecting: valueType)
         let comparator = comparator(for: T.self)
-        matchers.append((mirror, comparator as Any))
+        matchers.withValue { $0.append((mirror, comparator as Any)) }
     }
 }
 
@@ -181,7 +181,8 @@ extension Matcher {
 
 extension Matcher {
     private func comparator(by mirror: Mirror) -> Any? {
-        matchers.reversed().first { matcher -> Bool in
+        let snapshot = matchers.withValue { $0 }
+        return snapshot.reversed().first { matcher -> Bool in
             matcher.mirror.subjectType == mirror.subjectType
         }?.comparator
     }