Merge rc175-metal-integration into rc175: Metal Bragg integration engine on macOS; CPU profile fit in single precision with a shared exp (result-changing at ulp level on the CPU path only); CPU partial scaling loop reads its working copy in order

The CUDA engine is untouched: on the RTX 5080 p.hkl is byte-identical to rc175 on 4nwv, lysozyme
and cytochrome C. Under --no-gpu (lysozyme, cytochrome C, myoglobin) intensities move at the ulp
level from the single-precision fit and nothing changes beyond noise in battery compare; wall
44/117/71 s -> 41/105/65 s.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

common/CUDAWrapper.cpp

@@ -3,6 +3,7 @@
 
 #include "CUDAWrapper.h"
 
+#include <atomic>
 #include <condition_variable>
 #include <mutex>
 
@@ -93,7 +94,17 @@ int32_t get_gpu_count() {
     return 0;
 }
 
-void hide_gpus() {}
+namespace {
+    std::atomic<bool> gpus_hidden{false};
+}
+
+void hide_gpus() {
+    gpus_hidden = true;
+}
+
+bool gpus_are_hidden() {
+    return gpus_hidden;
+}
 
 std::vector<std::string> get_gpu_names() {
     return {};