Stop the dispatch tests measuring the CI runner instead of the loop

EffectiveConcurrentDownloads clamps the setting by Environment.ProcessorCount, so
a test asking for three books at once starts only two on a small machine, waits
out its ten second patience and fails. Reproduced with DOTNET_PROCESSOR_COUNT=2:
three failures and a thirty-four second run, against five seconds and none on a
sixteen core box. The CI matrix includes runners small enough to hit it.

Machine capability is now overridable, and the dispatch tests pin it to the
concurrency they are asking about. The clamp itself is unchanged in the app.

Two tests for the seam while it is there, since neither the point-of-use clamp
nor the hint had any coverage: that a machine smaller than the setting holds the
loop down without rewriting what the user asked for, and that ConcurrencyHint
says so and falls silent when the machine can keep up.

Source/LibationUiBase/ProcessQueue/ProcessQueueViewModel.cs

@@ -88,12 +88,22 @@ public class ProcessQueueViewModel : ReactiveObject
 	}
 	private int _maxConcurrentDownloads;
 
+	/// <summary>
+	/// What this machine can usefully manage. Overridable so that tests of the dispatch loop pin it
+	/// instead of inheriting the host's core count - otherwise a test asking for three books at once
+	/// passes on a developer's machine and fails on a two-core CI runner, having tested the runner
+	/// rather than the loop.
+	/// </summary>
+	internal int? MachineCeilingOverride { get; set; }
+
+	private int MachineCeiling => MachineCeilingOverride ?? Configuration.MaxAllowedConcurrentDownloads;
+
 	/// <summary>
 	/// How many books actually run at once: what the user asked for, held down to what this machine
 	/// can usefully manage. Applied here, at the point of use, so the stored setting is left alone.
 	/// </summary>
 	private int EffectiveConcurrentDownloads
-		=> Math.Clamp(MaxConcurrentDownloads, Configuration.MinConcurrentDownloads, Configuration.MaxAllowedConcurrentDownloads);
+		=> Math.Clamp(MaxConcurrentDownloads, Configuration.MinConcurrentDownloads, MachineCeiling);
 	public bool AutoScrollQueue { get => field; set { RaiseAndSetIfChanged(ref field, value); Configuration.Instance.AutoScrollQueue = value; } }
 
 	/// <summary>Exposed so UI controls can bind their spinner bounds rather than hardcoding them.</summary>