Fix readmeta(io, FatMachOHandle) for IOBuffers on 32-bit

`position()` of an `IOBuffer` is an `Int`, which on 32-bit platforms
does not match the `Int64` start of a `FatMachOHandle`.  Convert it,
as the other handle types already do.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: topolarity

src/MachO/MachOFat.jl

@@ -59,7 +59,7 @@ function readmeta(io::IO,::Type{FatMachOHandle})
 
     # Unpack the header
     header = unpack(io, header_type, endianness)
-    return FatMachOHandle(io, start, header, path(io))
+    return FatMachOHandle(io, Int64(start), header, path(io))
 end
 
 # Iteration

test/runtests.jl

@@ -207,7 +207,7 @@ function test_libfoo_and_fooifier(fooifier_path, libfoo_path)
 end
 
 function test_fat_libfoo(file)
-    ohs = readmeta(open(file, "r"))
+    ohs = readmeta(file isa IO ? file : open(file, "r"))
     @test isa(ohs, FatMachOHandle)
     @test length(ohs) == 2
     ntotal, n64 = 0, 0
@@ -245,7 +245,9 @@ test_libfoo_and_fooifier("./linux64/fooifier", "./linux64/libfoo.so")
 # Run MachO tests
 test_libfoo_and_fooifier("./mac64/fooifier", "./mac64/libfoo.dylib")
 test_fat_libfoo("./mac64/libfoo_fat.dylib")
+test_fat_libfoo(IOBuffer(read("./mac64/libfoo_fat.dylib")))
 test_fat_libfoo("./mac64/libfoo_fat64.dylib")
+test_fat_libfoo(IOBuffer(read("./mac64/libfoo_fat64.dylib")))
 test_metal("./macmetal/dummy")
 
 # Run COFF tests