ifcgit: conflict report panel and dry-run merge preview

Parse ifcmerge JSON output and display a per-conflict breakdown in the
panel when merge fails. Ctrl+click on the Merge button previews
conflicts without committing. Add SelectConflictEntity operator to
select and frame the conflicting object in the 3D viewport.

Generated with the assistance of an AI coding tool.

src/bonsai/test/tool/test_ifcgit.py

@@ -454,3 +454,128 @@ class TestIfcDiffIds(NewFile):
             result = IfcGit.ifc_diff_ids(repo, sha_a, sha_b, ifc_path)
             assert 1 in result["modified"]
             assert 2 in result["modified"]
+
+
+# ---------------------------------------------------------------------------
+# Merge conflict report — store / clear / get
+# ---------------------------------------------------------------------------
+
+
+class TestStoreClearGetMergeConflicts(NewFile):
+    def test_round_trip(self):
+        conflicts = [{"type": "attribute_conflict", "entity_id": 42}]
+        IfcGit.store_merge_conflicts(conflicts)
+        result = IfcGit.get_merge_conflicts()
+        assert result == conflicts
+
+    def test_get_returns_none_when_empty(self):
+        IfcGit.clear_merge_conflicts()
+        assert IfcGit.get_merge_conflicts() is None
+
+    def test_clear_removes_stored_conflicts(self):
+        IfcGit.store_merge_conflicts([{"type": "class_changed"}])
+        IfcGit.clear_merge_conflicts()
+        assert IfcGit.get_merge_conflicts() is None
+
+    def test_get_returns_none_on_corrupt_json(self):
+        import bpy
+
+        bpy.context.scene.IfcGitProperties.merge_conflicts = "not valid json {"
+        assert IfcGit.get_merge_conflicts() is None
+
+
+# ---------------------------------------------------------------------------
+# git_mergetool — report file reading
+# ---------------------------------------------------------------------------
+
+
+class TestGitMergetool:
+    @requires_git
+    def test_returns_none_when_report_file_absent(self):
+        import unittest.mock as mock
+
+        with tempfile.TemporaryDirectory() as tmpdir:
+            ifc_path = os.path.join(tmpdir, "model.ifc")
+            mock_repo = mock.MagicMock()
+            IfcGitRepo.repo = mock_repo
+            result = IfcGit.git_mergetool("ifcmerge", ifc_path)
+            assert result is None
+            IfcGitRepo.repo = None
+
+    @requires_git
+    def test_returns_none_when_report_file_empty(self):
+        import unittest.mock as mock
+
+        with tempfile.TemporaryDirectory() as tmpdir:
+            ifc_path = os.path.join(tmpdir, "model.ifc")
+            report_path = ifc_path + ".ifcmerge"
+            open(report_path, "w").close()
+            mock_repo = mock.MagicMock()
+            IfcGitRepo.repo = mock_repo
+            result = IfcGit.git_mergetool("ifcmerge", ifc_path)
+            assert result is None
+            assert not os.path.exists(report_path)
+            IfcGitRepo.repo = None
+
+    @requires_git
+    def test_parses_conflict_report_and_deletes_file(self):
+        import json
+        import unittest.mock as mock
+
+        with tempfile.TemporaryDirectory() as tmpdir:
+            ifc_path = os.path.join(tmpdir, "model.ifc")
+            report_path = ifc_path + ".ifcmerge"
+            conflicts = [{"type": "attribute_conflict", "entity_id": 5}]
+            with open(report_path, "w") as f:
+                json.dump({"status": "failed", "conflicts": conflicts}, f)
+            mock_repo = mock.MagicMock()
+            mock_repo.git.mergetool.side_effect = git.exc.GitCommandError("mergetool", 1)
+            IfcGitRepo.repo = mock_repo
+            result = IfcGit.git_mergetool("ifcmerge", ifc_path)
+            assert result == conflicts
+            assert not os.path.exists(report_path)
+            IfcGitRepo.repo = None
+
+
+# ---------------------------------------------------------------------------
+# config_ifcmerge — cmd format and update
+# ---------------------------------------------------------------------------
+
+
+class TestConfigIfcmerge:
+    @requires_git
+    def test_writes_redirect_cmd_on_first_call(self):
+        with tempfile.TemporaryDirectory() as tmpdir:
+            repo = _make_repo(tmpdir)
+            IfcGitRepo.repo = repo
+            IfcGit.config_ifcmerge()
+            reader = repo.config_reader()
+            cmd = reader.get_value('mergetool "ifcmerge"', "cmd")
+            assert "> $MERGED.ifcmerge" in cmd
+            IfcGitRepo.repo = None
+
+    @requires_git
+    def test_updates_cmd_missing_redirect(self):
+        with tempfile.TemporaryDirectory() as tmpdir:
+            repo = _make_repo(tmpdir)
+            IfcGitRepo.repo = repo
+            with repo.config_writer() as w:
+                w.set_value('mergetool "ifcmerge"', "cmd", "ifcmerge $BASE $LOCAL $REMOTE $MERGED")
+                w.set_value('mergetool "ifcmerge"', "trustExitCode", True)
+            IfcGit.config_ifcmerge()
+            reader = repo.config_reader()
+            cmd = reader.get_value('mergetool "ifcmerge"', "cmd")
+            assert "> $MERGED.ifcmerge" in cmd
+            IfcGitRepo.repo = None
+
+    @requires_git
+    def test_forward_tool_writes_redirect_cmd(self):
+        with tempfile.TemporaryDirectory() as tmpdir:
+            repo = _make_repo(tmpdir)
+            IfcGitRepo.repo = repo
+            IfcGit.config_ifcmerge()
+            reader = repo.config_reader()
+            cmd = reader.get_value('mergetool "ifcmerge-forward"', "cmd")
+            assert "--prioritise-local" in cmd
+            assert "> $MERGED.ifcmerge" in cmd
+            IfcGitRepo.repo = None