Fix arm64 build against current toolchain

[an1310]

Nestopia does not compile under current Xcode. Two issues:

• Several save-state routines brace-initialize byte arrays from wider
  integer expressions. C++11 makes narrowing in a braced initializer list
  ill-formed, and current clang enforces it. The truncations are
  intentional in serialization code, so -Wc++11-narrowing is disabled
  for the core target rather than casting each site.
• A state-serialization method declared to return NSData * returned
  NO on its error path. Changed to return nil.

Builds arm64 against the macOS 26 SDK.

[MaddTheSane]

Could you rebase this against master?

[an1310]

I kept the project default on the rebase. I was using static_cast, but the style used by this branch was a simple byte cast. (Also reverted spaces to tabs because that was the project default as well, though I've been using spaces for 20+ years)