Add Filter for D0 hfflag check similar to taskDplus

Following the pattern used in taskDplus, add a Filter to automatically
filter D0 candidates by decay type flag at the framework level.

Changes:
1. Add Filter filterD0Flag to check hfflag for D0ToPiK decay type
2. Wrap all D0 candidate types with soa::Filtered<> to apply the filter
3. Remove manual hfflag check in UPC process loop (now redundant)

This ensures the hfflag filtering is applied consistently across all
process functions and removes the need for manual checks in loops.

The Filter is placed after using declarations and before Preslice
declarations, following PWGHF struct member ordering guidelines.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: minjungkim12

PWGHF/D2H/Tasks/taskD0.cxx

@@ -110,22 +110,24 @@ struct HfTaskD0 {
   SliceCache cache;
   Service<o2::ccdb::BasicCCDBManager> ccdb;
 
-  using D0Candidates = soa::Join<aod::HfCand2Prong, aod::HfSelD0>;
-  using D0CandidatesMc = soa::Join<D0Candidates, aod::HfCand2ProngMcRec>;
-  using D0CandidatesKF = soa::Join<D0Candidates, aod::HfCand2ProngKF>;
-  using D0CandidatesMcKF = soa::Join<D0CandidatesKF, aod::HfCand2ProngMcRec>;
+  using D0Candidates = soa::Filtered<soa::Join<aod::HfCand2Prong, aod::HfSelD0>>;
+  using D0CandidatesMc = soa::Filtered<soa::Join<aod::HfCand2Prong, aod::HfSelD0, aod::HfCand2ProngMcRec>>;
+  using D0CandidatesKF = soa::Filtered<soa::Join<aod::HfCand2Prong, aod::HfSelD0, aod::HfCand2ProngKF>>;
+  using D0CandidatesMcKF = soa::Filtered<soa::Join<aod::HfCand2Prong, aod::HfSelD0, aod::HfCand2ProngKF, aod::HfCand2ProngMcRec>>;
 
-  using D0CandidatesMl = soa::Join<D0Candidates, aod::HfMlD0>;
-  using D0CandidatesMlMc = soa::Join<D0CandidatesMl, aod::HfCand2ProngMcRec>;
-  using D0CandidatesMlKF = soa::Join<D0CandidatesMl, aod::HfCand2ProngKF>;
-  using D0CandidatesMlMcKF = soa::Join<D0CandidatesMlKF, aod::HfCand2ProngMcRec>;
+  using D0CandidatesMl = soa::Filtered<soa::Join<aod::HfCand2Prong, aod::HfSelD0, aod::HfMlD0>>;
+  using D0CandidatesMlMc = soa::Filtered<soa::Join<aod::HfCand2Prong, aod::HfSelD0, aod::HfMlD0, aod::HfCand2ProngMcRec>>;
+  using D0CandidatesMlKF = soa::Filtered<soa::Join<aod::HfCand2Prong, aod::HfSelD0, aod::HfMlD0, aod::HfCand2ProngKF>>;
+  using D0CandidatesMlMcKF = soa::Filtered<soa::Join<aod::HfCand2Prong, aod::HfSelD0, aod::HfMlD0, aod::HfCand2ProngKF, aod::HfCand2ProngMcRec>>;
 
   using Collisions = soa::Join<aod::Collisions, aod::EvSels>;
   using CollisionsCent = soa::Join<aod::Collisions, aod::EvSels, aod::CentFT0Ms, aod::CentFT0Cs>;
   using CollisionsWithMcLabels = soa::Join<aod::Collisions, aod::McCollisionLabels, aod::EvSels>;
   using CollisionsWithMcLabelsCent = soa::Join<aod::Collisions, aod::McCollisionLabels, aod::EvSels, aod::CentFT0Ms, aod::CentFT0Cs>;
   using TracksSelQuality = soa::Join<aod::TracksExtra, aod::TracksWMc>;
 
+  Filter filterD0Flag = (o2::aod::hf_track_index::hfflag & static_cast<uint8_t>(BIT(aod::hf_cand_2prong::DecayType::D0ToPiK))) != static_cast<uint8_t>(0);
+
   Preslice<aod::HfCand2Prong> candD0PerCollision = aod::hf_cand::collisionId;
   PresliceUnsorted<CollisionsWithMcLabels> colPerMcCollision = aod::mccollisionlabel::mcCollisionId;
   PresliceUnsorted<CollisionsWithMcLabelsCent> colPerMcCollisionCent = aod::mccollisionlabel::mcCollisionId;
@@ -600,9 +602,6 @@ struct HfTaskD0 {
         float occ{-1.f};
 
         for (const auto& candidate : groupedD0Candidates) {
-          if (!(candidate.hfflag() & 1 << aod::hf_cand_2prong::DecayType::D0ToPiK)) {
-            continue;
-          }
           if (yCandRecoMax >= 0. && std::abs(HfHelper::yD0(candidate)) > yCandRecoMax) {
             continue;
           }