refactor(alert): move AHDC track-finding from AHDCEngine to ALERTEngine

[mathieuouillon]

AHDCEngine now only reads AHDC::adc, applies calibration via HitReader, and writes AHDC::hits. The full track-finding pipeline (preclustering, AI/CV_Distance/CV_Hough finder, DOCA refinement, helix fit) runs in ALERTEngine on top of AHDC::hits + ATOF::hits/clusters, alongside the existing projection / matching / prePID / Kalman steps.

The track finder is selected via the ALERT.Mode YAML key (was AHDC.Mode); ModelTrackFinding only loads when AI_Track_Finding is selected. The ATOF::tdc gate now fires after the AHDC pipeline so events without ATOF still get their AHDC::* banks, matching the pre-refactor behavior.

[mathieuouillon]

Note: AHDC::hits.adc is stored as int (calibrated ADC truncated). Track.get_sum_adc rounds Hit.getADC() per hit, so when track-finding ran inside AHDCEngine it summed the original full-precision double. Reading back from the bank here gives integer adc values, so sum_adc and dEdx in AHDC::track can drift by 0-1 per hit relative to the pre-refactor output (≈1-6 counts in sum_adc on ~1% of events). Eliminating the drift would require widening AHDC::hits.adc from I to F/D in the schema.

[mathieuouillon]

Compared AHDC::track row counts between the two finders on the same 1000-event input (clas_021903.evio.00000, recon-util -n 1000), toggling only the ALERT.Mode YAML key.

Metric	MLP	GNN
Total tracks	9	35
Mean tracks / event	0.009	0.035
Events with ≥1 track	9	34

GNN finds ~3.9× more tracks overall.

---

Per-event breakdown (1000 events)

Bucket	Count
Both finders have tracks	9
Only MLP has tracks	0
Only GNN has tracks	25
Neither has tracks	966
MLP rows > GNN rows (per event)	0
MLP rows < GNN rows (per event)	25
MLP rows == GNN rows	975