fix(vwb): propage l'image d'ancre aux substeps compound à l'import (SP-1/U-B)

Les actions compound passaient par _convert_compound_substep qui ne lisait
jamais l'image d'ancre du parent -> substeps anchor_id NULL, "Ancre requise"
sans image dans le VWB. On pose desormais l'ancre du parent (meme fallback que
la branche action simple) sur le 1er substep cliquable uniquement.

Test: test_learned_workflow_bridge.py (TDD, RED->GREEN).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

visual_workflow_builder/backend/services/learned_workflow_bridge.py

@@ -166,11 +166,30 @@ def convert_learned_to_vwb_steps(
                 )
                 continue
 
+            # Image d'ancre du parent : à poser sur le 1er substep cliquable.
+            # Les actions simples l'obtiennent déjà (branche `else` plus bas) ;
+            # les compound ne la propageaient pas → substeps anchor_id NULL.
+            compound_anchor_b64 = (
+                target.get("anchor_image_base64")
+                or target.get("screenshot")
+                or (target.get("context_hints") or {}).get("anchor_image_base64")
+            )
+            compound_anchor_attached = False
+
             for sub_idx, sub in enumerate(sub_steps):
                 sub_type = sub.get("type", "unknown")
                 sub_vwb_type, sub_params = _convert_compound_substep(
                     sub_type, sub, target
                 )
+                # Poser l'ancre du parent sur le 1er substep cliquable uniquement
+                # (éviter de la dupliquer sur les N substeps).
+                if (
+                    compound_anchor_b64
+                    and not compound_anchor_attached
+                    and sub_vwb_type in ("click_anchor", "double_click_anchor", "right_click_anchor")
+                ):
+                    sub_params["_anchor_image_base64"] = compound_anchor_b64
+                    compound_anchor_attached = True
                 label = _build_step_label(sub_vwb_type, sub_params, from_name, to_name)
                 steps.append({
                     "action_type": sub_vwb_type,