#3557 setup.sh: fail fast with a clear message on non-x86_64 (ARM) before downloading

Descripción

## What
Add a fail-fast architecture guard at the top of setup.sh so non-x86_64 machines (NVIDIA Jetson/AGX Xavier, Raspberry Pi, Apple Silicon Linux) get a clear message BEFORE downloading gigabytes of engine + CUDA/HIP wheels and crashing mid-install. Today setup.py only checks x64 via an x86_64-only CPUID stub, so ARM users find out the toolchain is x86_64-only too late.

## Why
Strata has no aarch64 build path (all prebuilt engine assets and NVIDIA/AMD wheels are x86_64; aarch64:0 and arm64:0 in setup.py; gpus()/CPUID assume x86_64). This guard turns the confusing late failure into an immediate, actionable message pointing to docs/OLDER_GPUS.md.

## Change
POSIX-sh block after cd "$(dirname "$0")":
  if [ "$(uname -m)" != "x86_64" ]; then
    echo "[X] Strata currently supports x86_64 only (this machine is $(uname -m))."
    echo "    Prebuilt engine and NVIDIA/AMD wheels are x86_64-only; no aarch64 path yet."
    echo "    Supported NVIDIA: RTX 20/30/40/50 (older Pascal/Volta via experimental CUDA-12, docs/OLDER_GPUS.md)."
    exit 1
  fi
Behavior on supported platforms is unchanged.

## Test
x86_64 passes; aarch64 prints clear error and exits 1 before any download. Happy to move the check into setup.py if maintainers prefer.