Fix fallback read too short for sparse prologues in redirection helper

Replace the two-attempt read with a single read sized to the smaller of:
- detourLength + 16 (the decoder's preferred window), and
- bytes remaining in the current page (so ReadProcessMemory does not fail whole read).

Reading only detourLength bytes could leave the instruction analyzer without enough
bytes to resolve a multi-byte instruction that crosses the splice point on
sparse prologues. Reading up to the page boundary gives the largest safe window.

Tests: 199 passing, 4 integration/interactive skipped.

WhiteMagic/Hooking/Detour.cs

@@ -66,18 +66,21 @@ public sealed class Detour : IDisposable
         int pointerSize = _memory.Is64Bit ? 8 : 4;
         int detourLength = pointerSize == 8 ? 14 : 5;
 
-        // Try to read detourLength + 16 bytes for the prologue decoder.
-        // If the target is near a page boundary, this might fail, so fall back to the minimum.
-        byte[] prologue = _memory.ReadBytes(Target, detourLength + 16);
+        // The prologue decoder may need to see bytes past the minimum detour length
+        // to identify the whole instruction that crosses the splice point. Prefer a
+        // generous read, but if the target sits near an unmapped page boundary, read
+        // only up to that boundary so ReadProcessMemory does not fail entirely.
+        int preferredBuffer = detourLength + 16;
+        int pageSize = Environment.SystemPageSize;
+        int pageOffset = (int)(Target.ToInt64() & (pageSize - 1));
+        int bytesToPageBoundary = pageSize - pageOffset;
+        int readSize = Math.Min(preferredBuffer, bytesToPageBoundary);
+
+        byte[] prologue = _memory.ReadBytes(Target, readSize);
         if (prologue.Length < detourLength)
         {
-            // Second attempt: read only the minimum required bytes
-            prologue = _memory.ReadBytes(Target, detourLength);
-            if (prologue.Length < detourLength)
-            {
-                throw new InvalidOperationException(
-                    "Could not read enough bytes from the target function to install a detour.");
-            }
+            throw new InvalidOperationException(
+                "Could not read enough bytes from the target function to install a detour.");
         }
 
         int preserveLength = PrologueDecoder.GetWholeInstructionLength(prologue, detourLength, _memory.Is64Bit);