Polish guest register state API

[zhangxp1998]

Summary

• Make GuestRegisterStateRef::write_gpr unsafe and return a Result for invalid register indexes.
• Replace the tuple-returning exception_return() helper with named return address/status accessors.
• Pass the saved non-volatile register frame as &mut NonVolatileRegisters instead of a raw pointer.
• Address the remaining PR Add stage-2 MMIO handler support #41 review comments: use LazyLock, use ArrayVec::is_full(), expose the remaining crate-local helpers as pub, and make read-result extension a DecodedMemoryAccess method.

Follow-up for review comments left on #41 after it was merged.

Tests

• cargo fmt --all -- --check
• git diff --check
• make build PLATFORM=qemu
• make test PLATFORM=qemu
• make clippy PLATFORM=qemu