Refactor downloaders to use ByteDocument and add options builders

Replaces generic RawType with ByteDocument in downloaders and context classes, simplifying type usage. Adds builder classes for FailurePredicateOptions, FragmentOptions, SkipPredicateOptions, and UnitDownloaderOptions to improve configuration flexibility. Introduces DownloadTarget enum and SkipPredicate delegate for more granular download control. Refactors Fluent API interfaces and implementations to remove RawType generics and streamline usage. Adds Playwright and Stealth download strategies for extensibility.

Beam.Downloaders/FailurePredicateOptionsBuilder.cs

@@ -0,0 +1,56 @@
+using Beam.Models;
+
+namespace Beam.Downloaders;
+
+public sealed class FailurePredicateOptionsBuilder<TRaw>
+{
+    private readonly System.Collections.Generic.List<AsyncDownloadFailurePredicate<TRaw>?> _predicates =
+        new System.Collections.Generic.List<AsyncDownloadFailurePredicate<TRaw>?>();
+    private bool _processInParallel = false;
+    private int? _parallelThreads = null;
+
+    public FailurePredicateOptionsBuilder<TRaw> WithPredicate(AsyncDownloadFailurePredicate<TRaw>? predicate)
+    {
+        _predicates.Add(predicate);
+        return this;
+    }
+
+    public FailurePredicateOptionsBuilder<TRaw> WithPredicates(System.Collections.Generic.IEnumerable<AsyncDownloadFailurePredicate<TRaw>?> predicates)
+    {
+        if (predicates == null) throw new System.ArgumentNullException(nameof(predicates));
+        _predicates.AddRange(predicates);
+        return this;
+    }
+
+    public FailurePredicateOptionsBuilder<TRaw> WithPredicates(params AsyncDownloadFailurePredicate<TRaw>?[] predicates)
+    {
+        _predicates.Clear();
+        if (predicates != null) _predicates.AddRange(predicates);
+        return this;
+    }
+
+    public FailurePredicateOptionsBuilder<TRaw> WithProcessInParallel(bool value = true)
+    {
+        _processInParallel = value;
+        return this;
+    }
+
+    public FailurePredicateOptionsBuilder<TRaw> WithParallelThreads(int? threads)
+    {
+        if (threads.HasValue && threads.Value <= 0)
+            throw new System.ArgumentOutOfRangeException(nameof(threads));
+        _parallelThreads = threads;
+        return this;
+    }
+
+    public FailurePredicateOptions<TRaw> Build()
+    {
+        var arr = _predicates.Count == 0 ? [] : _predicates.ToArray();
+        return new FailurePredicateOptions<TRaw>
+        {
+            AsyncDownloadFailurePredicates = arr,
+            ProcessInParallel = _processInParallel,
+            ParallelThreads = _parallelThreads
+        };
+    }
+}