viewer: drop the cached pixel mask when switching HTTP source

cached_pixel_mask is keyed only on arm_date and was cleared only in
Close(); ReadURL() reused it, so switching to a source whose start message
has an empty or duplicate arm_date silently kept the previous source's
mask (wrong pixels masked in the live preview and fed to spot-finding /
azimuthal integration). Clear it in ReadURL() too, as Close() does.

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

viewer/JFJochHttpReader.cpp

@@ -298,6 +298,10 @@ std::shared_ptr<JFJochReaderDataset> JFJochHttpReader::UpdateDataset_i() {
 void JFJochHttpReader::ReadURL(const std::string &url) {
     std::unique_lock ul(http_mutex);
     ResetConnection();  // selecting an address (re)opens the connection from scratch
+    // Drop the previous source's cached mask: a different (or re-armed) source has its own mask, and
+    // the arm_date key alone does not distinguish an empty/duplicate arm_date across sources.
+    cached_pixel_mask.reset();
+    cached_pixel_mask_arm_date.clear();
     addr = url;
     if (url.empty())
         SetStartMessage({});