code-to10 v4 coverage thickening: 219 new assertion tests (batches A/B1/C/D1-D6)

Baseline merge head e5ff26a: 28144 stmts, 2807 missing, exact 90.0263%, margin +7.
After thickening (full run): 2350 missing, exact 91.6501%, margin +464.
Full suite 3516 items: 3509 pass, 1 failure (pre-existing November date
failure, untouched), 6 skipped. No production code changed.

tests/test_v2cov_gaps_d6.py

@@ -0,0 +1,104 @@
+"""Coverage batch v4-D6: host.py parallel-branch fallback paths
+(4186 dep-branch dispatch failure, 4198 dep cwd copy).
+
+Stubbing discipline: the branch code under test is never stubbed.
+Patched collaborators are dispatch, dispatch_from_branch,
+run_task_turn_with_retry and copy_tree_excluding_git (all of them
+sinks the branch delegates to). Assertions read real stdout and the
+real copy_tree call record.
+"""
+
+import contextlib
+import io
+import json
+import os
+import sys
+import tempfile
+import unittest
+from pathlib import Path
+from unittest import mock
+
+REPO_ROOT = Path(__file__).resolve().parents[1]
+sys.path.insert(0, str(REPO_ROOT))
+
+import host
+
+
+class _FakeServer:
+    def __init__(self, *args, **kwargs):
+        pass
+
+    def initialize(self):
+        pass
+
+    def set_trace_file(self, *args, **kwargs):
+        pass
+
+    def close(self):
+        pass
+
+
+class ParallelFallbackTestCase(unittest.TestCase):
+    def test_dep_branch_dispatch_falls_back_to_head_plus_copy(self):
+        # 4186 + 4198
+        with tempfile.TemporaryDirectory() as tmp:
+            dep_dir = os.path.join(tmp, "dep-cwd")
+            os.makedirs(dep_dir)
+            with open(os.path.join(dep_dir, "seed.txt"), "w") as f:
+                f.write("seed")
+            new_dir = os.path.join(tmp, "new-cwd")
+            jobs_path = os.path.join(tmp, "jobs.json")
+            jobs = [
+                {"name": "j1", "task": "t1", "repo": "/tmp/repo1"},
+                {"name": "j2", "task": "t2", "repo": "/tmp/repo1",
+                 "depends_on": ["j1"]},
+            ]
+            with open(jobs_path, "w", encoding="utf-8") as f:
+                json.dump(jobs, f)
+            state_path = os.path.join(tmp, "host-state.json")
+            cfg_path = os.path.join(tmp, "host-config.json")
+            with open(state_path, "w", encoding="utf-8") as f:
+                json.dump({"tasks": {}, "automations": []}, f)
+            with open(cfg_path, "w", encoding="utf-8") as f:
+                json.dump({}, f)
+
+            def _dispatch(srv, state, name, task, repo, *a, **k):
+                if name == "j1":
+                    return {"name": "j1", "status": "completed",
+                            "branch": "b1", "cwd": dep_dir}
+                return {"name": "j2", "status": "completed",
+                        "cwd": new_dir}
+
+            copy_calls = []
+            argv = ["host.py", "--state", state_path,
+                    "--config", cfg_path, "parallel",
+                    "--jobs", jobs_path]
+            buf = io.StringIO()
+            with contextlib.redirect_stdout(buf):
+                with mock.patch.object(sys, "argv", argv):
+                    with mock.patch.object(
+                            host, "Server",
+                            lambda **kw: _FakeServer()):
+                        with mock.patch.object(
+                                host, "dispatch",
+                                side_effect=_dispatch):
+                            with mock.patch.object(
+                                    host, "dispatch_from_branch",
+                                    return_value=(None, "boom")):
+                                with mock.patch.object(
+                                        host,
+                                        "run_task_turn_with_retry",
+                                        return_value={"status": "completed",
+                                                      "text": "ok"}):
+                                    with mock.patch.object(
+                                            host,
+                                            "copy_tree_excluding_git",
+                                            side_effect=lambda s, d: copy_calls.append((s, d))):
+                                        host.main()
+            out = buf.getvalue()
+            self.assertIn("从依赖分支派发失败", out)
+            self.assertEqual(copy_calls, [(dep_dir, new_dir)])
+
+
+if __name__ == "__main__":
+    unittest.main()