fix(hwfit): normalize CPU arch for fallback estimates (#4441)

tests/test_hwfit_cpu_only_fallback.py

@@ -47,6 +47,12 @@ ARM64_SYSTEM = {
     "gpu_vram_gb": 0,
 }
 
+ARM32_SYSTEM = {
+    "backend": "arm",
+    "gpu_name": None,
+    "gpu_vram_gb": 0,
+}
+
 AARCH64_SYSTEM = {
     "backend": "aarch64",
     "gpu_name": None,
@@ -79,6 +85,16 @@ def test_cpu_only_on_metal_apple_silicon_uses_cpu_arm_fallback():
     assert metal_tps > 0
 
 
+def test_cpu_only_on_gpu_backend_uses_detected_arm64_cpu_arch():
+    """A GPU backend on an ARM64 host should use the ARM CPU fallback for cpu_only."""
+    cuda_arm64 = dict(CUDA_SYSTEM, cpu_arch="aarch64", cpu_name="Ampere Altra")
+    cuda_arm64_tps = _estimate_speed(DENSE_MODEL, QUANT, "cpu_only", cuda_arm64)
+    arm_tps = _estimate_speed(DENSE_MODEL, QUANT, "cpu_only", CPU_ARM_SYSTEM)
+
+    assert cuda_arm64_tps == pytest.approx(arm_tps, rel=1e-9, abs=1e-9)
+    assert cuda_arm64_tps > 0
+
+
 @pytest.mark.parametrize(
     "arm_alias_system",
     [ARM64_SYSTEM, AARCH64_SYSTEM, CPU_ARM_SYSTEM],
@@ -93,6 +109,15 @@ def test_cpu_only_preserves_arm_backends(arm_alias_system):
     assert alias_tps > 0
 
 
+def test_cpu_only_does_not_treat_plain_arm_as_arm64_fallback():
+    """Docker/OCI plain arm is not the ARM64-class fallback used for Apple Silicon."""
+    arm32_tps = _estimate_speed(DENSE_MODEL, QUANT, "cpu_only", ARM32_SYSTEM)
+    x86_tps = _estimate_speed(DENSE_MODEL, QUANT, "cpu_only", CPU_X86_SYSTEM)
+
+    assert arm32_tps == pytest.approx(x86_tps, rel=1e-9, abs=1e-9)
+    assert arm32_tps > 0
+
+
 def test_cpu_only_preserves_known_cpu_backends():
     """Known CPU backends should be preserved, not rewritten to cpu_x86."""
     for system in (CPU_X86_SYSTEM, CPU_ARM_SYSTEM):