core: report what occupies a fixed-address allocation on failure (#278)

* Implement DescribeAddressForDiagnostics method

Added a method to describe the state of a memory address for diagnostics.

* core: enrich allocation failure exception with host region diagnostic

src/SharpEmu.Core/Loader/SelfLoader.cs

@@ -197,8 +197,9 @@ public sealed class SelfLoader : ISelfLoader
             {
                 if (!physicalVm.TryAllocateAtExact(imageBase, totalImageSize, executable: true, out var allocatedBase))
                 {
+                    var reason = physicalVm.DescribeAddressForDiagnostics(imageBase);
                     throw new InvalidOperationException(
-                        $"Could not allocate main image at required base 0x{imageBase:X16} (size=0x{totalImageSize:X}).");
+                        $"Could not allocate main image at required base 0x{imageBase:X16} (size=0x{totalImageSize:X}): {reason}.");
                 }
 
                 imageBase = allocatedBase;

src/SharpEmu.Core/Memory/PhysicalVirtualMemory.cs

@@ -198,6 +198,24 @@ public sealed unsafe class PhysicalVirtualMemory : IVirtualMemory, IGuestMemoryA
         return true;
     }
 
+    public string DescribeAddressForDiagnostics(ulong address)
+    {
+        if (!_hostMemory.Query(address, out var info))
+        {
+            return "unable to query host memory at this address";
+        }
+
+        return info.State switch
+        {
+            HostRegionState.Free => "address reports free, but the exact-address reservation still failed",
+            HostRegionState.Reserved =>
+                $"already reserved by another host allocation (base=0x{info.AllocationBase:X16}, size=0x{info.RegionSize:X})",
+            HostRegionState.Committed =>
+                $"already committed by another host allocation (base=0x{info.AllocationBase:X16}, size=0x{info.RegionSize:X}, protect=0x{info.RawProtection:X})",
+            _ => $"in an unexpected host state (raw=0x{info.RawState:X})",
+        };
+    }
+
     public ulong AllocateAt(ulong desiredAddress, ulong size, bool executable = true, bool allowAlternative = true)
     {
         if (size == 0)