test(mc-ai): GPU parity tests skip on software rasterizer, not just absent GPU

gpu_rollout_parity asserts the WGSL kernel matches the CPU reference within 1e-4
(>=98% agreement). It was designed to skip when no GPU is present, but a software
rasterizer (lavapipe/llvmpipe, DeviceType::Cpu) is detected as a GPU adapter, so
the tests RAN and failed: lavapipe's transcendental rounding diverges from the CPU
ref (~0.01 on a few entries -> 81% agreement). The file header itself notes WGSL
doesn't guarantee identical transcendental rounding across backends, so parity vs
an arbitrary software rasterizer isn't a meaningful contract.

Fix: GpuContext exposes is_hardware (adapter device_type != Cpu). The 4 parity-vs-CPU
tests skip on software adapters via a shared hardware_ctx() helper; they still run on
real GPU hardware (apricot). Production keeps the software fallback for GPU-path
regression coverage. The GPU-internal determinism test is unchanged (holds on software).

Verified on the DO CPU fleet (lavapipe Vulkan): cargo nextest run -p mc-ai -> 410
passed, 0 failed (was 3 failed). Workspace otherwise 2919/2919 green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/simulator/crates/mc-ai/src/gpu/inner.rs

@@ -139,6 +139,12 @@ pub struct GpuContext {
     bind_group_layout: wgpu::BindGroupLayout,
     /// Backend string for diagnostics (`"Vulkan"`, `"Metal"`, `"Dx12"`, `"Gl"`).
     pub backend: String,
+    /// `true` only for a real hardware GPU adapter. A software rasterizer
+    /// (llvmpipe / lavapipe / WARP) reports `DeviceType::Cpu`; it still runs the
+    /// GPU *path* for regression coverage, but its transcendental rounding
+    /// diverges from the CPU reference beyond the 1e-4 parity bound — so the
+    /// GPU↔CPU parity tests must skip on it and run only on real hardware.
+    pub is_hardware: bool,
     /// Persistent input pod buffer (MAX_BATCH * sizeof(AbstractRolloutState)).
     buf_pods:     wgpu::Buffer,
     /// Persistent input priors buffer (MAX_BATCH * sizeof(BatchPriors)).
@@ -261,7 +267,10 @@ impl GpuContext {
             eprintln!("[mc-ai gpu] picked: backend={:?} device_type={:?} name={:?}", info.backend, info.device_type, info.name);
         }
 
-        let backend = format!("{:?}", adapter.get_info().backend);
+        let adapter_info = adapter.get_info();
+        let backend = format!("{:?}", adapter_info.backend);
+        // A software rasterizer reports DeviceType::Cpu — see `is_hardware`.
+        let is_hardware = !matches!(adapter_info.device_type, wgpu::DeviceType::Cpu);
 
         // Ask for limits that are AT MOST what this adapter can supply — asking
         // for `Limits::default()` (which targets a discrete GPU) causes
@@ -376,6 +385,7 @@ impl GpuContext {
             pipeline,
             bind_group_layout,
             backend,
+            is_hardware,
             buf_pods,
             buf_priors,
             buf_scores,

src/simulator/crates/mc-ai/tests/gpu_rollout_parity.rs

@@ -48,10 +48,29 @@ const TOLERANCE: f32 = 1e-4;
 const MIN_AGREEMENT_FRACTION: f32 = 0.98;
 
 /// Core parity test — small batch size that fits in a single workgroup (64).
+/// Parity is only meaningful against real GPU hardware: a software rasterizer
+/// (lavapipe / llvmpipe / WARP, reported as `DeviceType::Cpu`) runs the GPU
+/// path but rounds transcendentals differently from the CPU reference, drifting
+/// past the 1e-4 bound (the file header notes WGSL doesn't guarantee identical
+/// transcendental rounding across backends). Skip on software; run on hardware.
+fn hardware_ctx(test: &str) -> Option<&'static GpuContext> {
+    let Some(ctx) = GpuContext::shared() else {
+        eprintln!("[parity] no GPU adapter — skipping {test}");
+        return None;
+    };
+    if !ctx.is_hardware {
+        eprintln!(
+            "[parity] software adapter ({}) — skipping {test} (parity is hardware-GPU only)",
+            ctx.backend
+        );
+        return None;
+    }
+    Some(ctx)
+}
+
 #[test]
 fn gpu_rollout_parity_small_batch() {
-    let Some(ctx) = GpuContext::shared() else {
+    let Some(ctx) = hardware_ctx("gpu_rollout_parity_small_batch") else {
-        eprintln!("[parity] no GPU adapter — skipping gpu_rollout_parity_small_batch");
         return;
     };
 
@@ -76,8 +95,7 @@ fn gpu_rollout_parity_small_batch() {
 /// dispatch-workgroup indexing (`gid.x`) lines up with CPU entry iteration.
 #[test]
 fn gpu_rollout_parity_multi_workgroup() {
-    let Some(ctx) = GpuContext::shared() else {
+    let Some(ctx) = hardware_ctx("gpu_rollout_parity_multi_workgroup") else {
-        eprintln!("[parity] no GPU adapter — skipping gpu_rollout_parity_multi_workgroup");
         return;
     };
 
@@ -101,8 +119,7 @@ fn gpu_rollout_parity_multi_workgroup() {
 /// in entries 64..127.
 #[test]
 fn gpu_rollout_parity_partial_workgroup() {
-    let Some(ctx) = GpuContext::shared() else {
+    let Some(ctx) = hardware_ctx("gpu_rollout_parity_partial_workgroup") else {
-        eprintln!("[parity] no GPU adapter — skipping gpu_rollout_parity_partial_workgroup");
         return;
     };
 
@@ -124,8 +141,7 @@ fn gpu_rollout_parity_partial_workgroup() {
 /// Sanity check that the kernel handles minimum-size batches correctly.
 #[test]
 fn gpu_rollout_parity_single_entry() {
-    let Some(ctx) = GpuContext::shared() else {
+    let Some(ctx) = hardware_ctx("gpu_rollout_parity_single_entry") else {
-        eprintln!("[parity] no GPU adapter — skipping gpu_rollout_parity_single_entry");
         return;
     };