miniCBF: read the sweep's rotation angles on several threads

Opening a sweep parses the header of every file to get its start angle, and one
header is two dozen std::regex searches: measured at 0.33 ms on a 16 Mpx sweep
and 1.0 ms on one with a longer header, so a 7200-frame sweep spent 7.2 s on one
thread before it read an image. The files are independent, so the scan is shared
out over up to eight of them.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

reader/JFJochCBFReader.cpp

@@ -8,8 +8,10 @@
 #include <cstring>
 #include <filesystem>
 #include <array>
+#include <future>
 #include <map>
 #include <optional>
+#include <thread>
 
 #include "../common/JFJochException.h"
 #include "../common/Logger.h"
@@ -258,10 +260,21 @@ void JFJochCBFReader::ReadFiles(const std::string &path) {
         std::chrono::duration_cast<std::chrono::nanoseconds>(
             std::chrono::duration<double>(header0_.exposure_s)));
 
-    // The rotation angle of every image, from its own header.
+    // The rotation angle of every image, from its own header. Parsing one costs a few hundred
+    // microseconds - it is two dozen regular expressions - so on a sweep of several thousand frames
+    // this is seconds of startup before a single image is read, and the files are independent.
     std::vector<double> angles(files_.size());
-    for (size_t i = 0; i < files_.size(); i++)
-        angles[i] = minicbf::ReadHeader(files_[i]).start_angle_deg;
+    {
+        const size_t nthreads = std::min<size_t>(std::max(1u, std::thread::hardware_concurrency()), 8);
+        std::vector<std::future<void>> futures;
+        for (size_t t = 0; t < nthreads; t++)
+            futures.push_back(std::async(std::launch::async, [&, t] {
+                for (size_t i = t; i < files_.size(); i += nthreads)
+                    angles[i] = minicbf::ReadHeader(files_[i]).start_angle_deg;
+            }));
+        for (auto &f : futures)
+            f.get();
+    }
 
     double increment = header0_.angle_increment_deg;
     if (files_.size() > 1) {