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.HLE/IGuestAddressSpace.cs

@@ -15,6 +15,17 @@ public interface IGuestAddressSpace : IGuestMemoryAllocator
 {
     ulong AllocateAt(ulong desiredAddress, ulong size, bool executable = true, bool allowAlternative = true);
 
+    /// <summary>
+    /// Backs an entire fixed-address range, matching the guest's
+    /// <c>SCE_KERNEL_MAP_FIXED</c> contract. Unlike <see cref="AllocateAt"/>, which
+    /// reserves the range in one all-or-nothing host call, this walks the range and
+    /// fills only the sub-ranges that are not already backed. That keeps a fixed
+    /// mapping whole when part of the requested window is already occupied — the
+    /// partial-overlap case where the single-call reservation fails outright and
+    /// leaves the remainder unmapped for the guest to fault into.
+    /// </summary>
+    bool TryBackFixedRange(ulong address, ulong size, bool executable);
+
     bool TryAllocateAtOrAbove(ulong desiredAddress, ulong size, bool executable, ulong alignment, out ulong actualAddress);
 
     bool TryProtect(ulong address, ulong size, GuestPageProtection protection);