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.Stealth/Strategies/WaitingDownloadStrategy.cs

@@ -0,0 +1,83 @@
+using System.Diagnostics;
+using Beam.Abstractions;
+using Beam.Models;
+using Microsoft.Extensions.Logging;
+using File = System.IO.File;
+
+namespace Beam.Stealth.Strategies;
+
+public class WaitingDownloadStrategy : IDownloadStrategy {
+    public async Task DownloadToStream(string url, int bufferSize, Stream destinationStream, IProgress<IDownloadReport> progress, StealthConfig config,
+        ILogger? logger, CancellationToken ct) {
+        await using var stream = await WaitForDownloadAsync(url, progress, Stopwatch.StartNew(), config, logger, ct);
+        await (stream?.CopyToAsync(destinationStream, ct) ?? Task.CompletedTask);
+    }
+    
+    
+        private async Task<Stream?> WaitForDownloadAsync(
+            string link, IProgress<IDownloadReport> progress, Stopwatch sw, StealthConfig config, ILogger? logger, CancellationToken ct) {
+            const int PollDelayMs = 250;   // how often we look
+            const int StableDelayMs = 1000;  // size-unchanged window
+
+            string dir = config.DownloadsDirectory;
+            string? finalPath = null;
+            long lastSize = -1;
+            DateTime lastChange = DateTime.UtcNow;
+
+            bool IsTemp(string p) =>
+                p.EndsWith(".crdownload", StringComparison.OrdinalIgnoreCase) ||
+                p.EndsWith(".part", StringComparison.OrdinalIgnoreCase);
+
+            logger?.LogDebug("Polling {Dir} for download files", dir);
+
+            while (sw.Elapsed < config.TimeOut && !ct.IsCancellationRequested) {
+                // current files in the directory
+                var files = Directory.EnumerateFiles(dir, "*", SearchOption.TopDirectoryOnly).ToArray();
+
+                // ignore temp names; pick (or re-pick) the first real candidate
+                finalPath ??= files.FirstOrDefault(f => !IsTemp(f));
+
+                // still nothing but temps – keep waiting
+                if (finalPath is null) {
+                    await Task.Delay(PollDelayMs, ct);
+                    continue;
+                }
+
+                // track growth
+                long size = new FileInfo(finalPath).Length;
+                if (size == 0 || size != lastSize) {
+                    progress?.Report(new DownloadReport() {
+                        BytesDownloaded = size - lastSize,
+                    });
+                    lastSize = size;
+                    lastChange = DateTime.UtcNow;
+                    await Task.Delay(PollDelayMs, ct);
+                    continue;
+                }
+
+                // size stable long enough *and* no temp files left?
+                bool tempsRemain = files.Any(IsTemp);
+                if ((DateTime.UtcNow - lastChange).TotalMilliseconds < StableDelayMs || tempsRemain) {
+                    await Task.Delay(PollDelayMs, ct);
+                    continue;
+                }
+
+                // wait until writer releases lock
+                while (true) {
+                    try {
+                        using FileStream _ =
+                            File.Open(finalPath, FileMode.Open, FileAccess.Read, FileShare.None);
+                        break;
+                    } catch (IOException) {
+                        await Task.Delay(200, ct);
+                    }
+                }
+
+                return File.OpenRead(finalPath);
+            }
+
+            logger?.LogWarning("Download timed out after {Elapsed}", sw.Elapsed);
+            return null;
+        }
+
+}