tests: CRH sections + DP diag bonus + case 74 regression + fusion propagation

- test_extraction: +21 tests (sections diag_sortie/diag_principal/synthese,
  variantes titres, terminaisons, faux positifs mid-sentence, biosynthèse)
- test_dp_selector: +55 tests (flags, candidates, scoring, hardening DIM,
  bonus +4/+2, evidence excerpt, cas 74 D50→I25.1 corrigé)
- test_fusion: +39 tests (propagation dp_selection evidence/reason/verdict,
  source 2e dossier, pas de crash si aucun DP)
- fixtures: case_74_min.json + 3 fixtures DP existantes

Aucun mock utilisé — données synthétiques uniquement.
Le test cas 74 passe : I25.1 gagne sur D50 grâce au bonus diag_sortie +4.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/resources/dp_ambiguous.json

@@ -0,0 +1,29 @@
+{
+  "description": "Ambigu : deux pathologies aigues de score similaire → REVIEW",
+  "dossier": {
+    "document_type": "crh",
+    "sejour": {"sexe": "M", "age": 55, "duree_sejour": 6},
+    "diagnostic_principal": null,
+    "diagnostics_associes": [
+      {
+        "texte": "Pneumopathie infectieuse",
+        "cim10_suggestion": "J18.9",
+        "cim10_confidence": "high",
+        "source": "llm_das"
+      },
+      {
+        "texte": "Insuffisance cardiaque décompensée",
+        "cim10_suggestion": "I50.1",
+        "cim10_confidence": "high",
+        "source": "llm_das"
+      }
+    ]
+  },
+  "synthese_nuke1": {
+    "motif": "Dyspnée fébrile",
+    "conclusion": "Décompensation cardiaque sur pneumopathie infectieuse"
+  },
+  "expected": {
+    "verdict": "REVIEW"
+  }
+}