Merge branch 'sg-arms' into rc175: every merge starts from G = 1; the all-observation arm merged and searched beside the first

Exact: P4 p.hkl identical to the base on 39 noise-subset sets; concurrent arms p.hkl md5 and MTZ data identical
to P4 on 12 sets at default -N and -N 8 (incl. 5 sets where the arms disagree). Side arm on card 2 with 3+ cards,
else on the last card when 2 x observations x 256 B fits its total memory (decided up front).

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SVmAWnzCmRKAXVUCdc4iNi

common/CUDAWrapper.cpp

@@ -61,6 +61,10 @@ std::vector<std::string> get_gpu_names() {
     return {};
 }
 
+size_t get_gpu_total_memory(int32_t) {
+    return 0;
+}
+
 void set_gpu(int32_t dev_id) {}
 
 void pin_gpu() {}