memory: back the free pages of a partially-overlapping fixed mapping (#458)

A SCE_KERNEL_MAP_FIXED request whose window partially overlaps an
existing allocation was failing outright: AllocateAt reserves the whole
range in one all-or-nothing VirtualAlloc, which returns 0 on partial
overlap. The mapping call then returned NOT_FOUND while leaving the free
tail unmapped, so the guest faulted (0xC0000005) writing into it.

Add IGuestAddressSpace.TryBackFixedRange, which walks the range via the
host Query (VirtualQuery reports contiguous same-state runs) and fills
only the free sub-ranges, leaving already-backed pages untouched. This
matches the fixed-mapping contract on hardware. Route the fixed
reservation path through it via a new backPartialOverlap flag.

Co-authored-by: slick-daddy <slick-daddy@users.noreply.github.com>

src/SharpEmu.Libs/Kernel/KernelMemoryCompatExports.cs

@@ -4603,7 +4603,8 @@ public static partial class KernelMemoryCompatExports
             allowSearch: false,
             allowAllocateAtAlternative: false,
             "reserve fixed range",
-            out _);
+            out _,
+            backPartialOverlap: true);
     }
 
     internal static bool IsGuestRangeBacked(CpuContext ctx, ulong address, ulong length)