viewer: overlay one plot line per processing run (with editable names)

The dataset-info plot now overlays every run as a separate named line instead of
replacing the plot when a snapshot is activated:

- Reader gains AllSnapshotDatasets() (every snapshot's dataset, Original first).
- The worker owns the run collection: a stable snapshot id plus an editable
  display label (RunData), emitted as runsChanged(runs, active_id) on file open,
  snapshot register, activate and rename. RenameRun(id, label) updates the legend
  label without touching the reader key (decoupled id vs label).
- The chart view draws the active run as the primary series (markers, hover,
  binning, axes) and the other runs as overlay lines sharing its axes, with a
  legend shown when overlaying (appendSeries factored out for both).
- The dataset-info widget holds the run list + the live run, extracts the selected
  metric from each (ExtractMetric), and unions the available metrics across runs.
- The live run is its own "Live" overlay (lightweight per-tick refresh, no combo
  rebuild); it is cleared on finish so the persisted snapshot takes over.
- The processing jobs table gets a "Started" column and an editable Name column;
  editing a name renames that run's legend label.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

reader/JFJochHDF5Reader.cpp

@@ -178,3 +178,17 @@ std::string JFJochHDF5Reader::ActiveSnapshot() const {
     std::unique_lock ul(hdf5_mutex);
     return active_snapshot_;
 }
+
+std::vector<std::pair<std::string, std::shared_ptr<const JFJochReaderDataset>>>
+JFJochHDF5Reader::AllSnapshotDatasets() const {
+    std::unique_lock ul(hdf5_mutex);
+    std::vector<std::pair<std::string, std::shared_ptr<const JFJochReaderDataset>>> out;
+    out.reserve(snapshots_.size());
+    // "Original" first, then the rest, so overlay colours stay stable across updates.
+    if (auto it = snapshots_.find("Original"); it != snapshots_.end())
+        out.emplace_back(it->first, it->second->Dataset());
+    for (const auto &[name, source]: snapshots_)
+        if (name != "Original")
+            out.emplace_back(name, source->Dataset());
+    return out;
+}

reader/JFJochHDF5Reader.h

@@ -63,4 +63,7 @@ public:
     void SetActiveSnapshot(const std::string &name);
     std::vector<std::string> SnapshotNames() const;
     std::string ActiveSnapshot() const;
+
+    // All snapshot datasets (name -> dataset), for overlaying every run's plots at once.
+    std::vector<std::pair<std::string, std::shared_ptr<const JFJochReaderDataset>>> AllSnapshotDatasets() const;
 };