Fix LibraryLoader crash on error during context-transfer restoration

- Track whether transferred thread's original context was successfully restored.
- In the catch block, leak the remote allocation instead of freeing it if the thread was not restored; this prevents the target process from executing freed memory.
- Remove redundant 'op == 0x55' check in InstructionAnalyzer (already matched by (op & 0xF8) == 0x50).
- Simplify MemoryBase ReadString align-down expression to previousLen - (previousLen % nullLen).
- Change StubAllocator size parameter from nint to int for clarity (internal test seam).

Tests: 199 passing, 4 integration/interactive skipped.

WhiteMagic/Execution/RemoteThreadExecutor.cs

@@ -41,7 +41,7 @@ public sealed class RemoteThreadExecutor
     /// When this delegate returns a non-zero pointer, the executor does not take
     /// ownership of that memory and will not free it.
     /// </remarks>
-    internal Func<IntPtr, nint, IntPtr>? StubAllocator { get; set; }
+    internal Func<IntPtr, int, IntPtr>? StubAllocator { get; set; }
 
     /// <summary>Test seam: overrides remote scratch allocation for string/struct args.
     /// Defaults to <see cref="NativeMethods.VirtualAllocEx"/>.</summary>