Clone the registry GTouchable per step to stop cross-event state leak

getGTouchable returned the persistent shared_ptr<GTouchable> stored in
gTouchableMap, and ProcessHits/processTouchable mutate it each step
(trackId, pid, stepTimeAtElectronicsIndex). Initialize() clears the
per-event touchableVector but never resets those mutations, so at the
start of each event the registry object still holds the previous event's
time-cell index. processTouchableImpl branches on that index, so the
first step of a new event can be misrouted (a spurious clone / wrong time
cell), and stale trackId/pid carry over.

Return a per-step copy so the registry entry stays pristine. Each step
then starts from the clean UNSET identity, and within-event grouping
still works because the hit collection / touchableVector group by
operator== over field values (which are freshly assigned per step), not
by object identity. gsd.h documents instances as thread-local with a
per-worker map, so the clone introduces no cross-thread sharing.

Verified: cherenkov example (gPhotonDetector SD, 20 events, fixed seed)
produces byte-identical hit physics before and after (no regression); the
fix removes the cross-event index carryover that affects readout SDs
(Geant4 11.4.1 dev container).

Fixes #113

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: zhaozhiwen

gemc/gsd/gsd.h

@@ -195,7 +195,9 @@ class GSensitiveDetector : public GBase<GSensitiveDetector>, public G4VSensitive
 		std::string vname = thisStep->GetPreStepPoint()->GetTouchable()->GetVolume()->GetName();
 
 		auto it = gTouchableMap.find(vname);
-		if (it != gTouchableMap.end()) { return it->second; }
+		// Return a per-step copy: ProcessHits and processTouchable mutate trackId, pid and the
+		// time-cell index, so the registry entry must stay pristine across steps and events.
+		if (it != gTouchableMap.end()) { return std::make_shared<GTouchable>(*it->second); }
 		// If not found, log an error. The calling code assumes a valid pointer.
 		log->error(ERR_DYNAMICPLUGINNOTFOUND, "GTouchable for volume " + vname + " not found in gTouchableMap");
 	}