fix: kill the whole process tree on Windows spawn-test timeout (#13)

Popen.kill() only terminated the direct child, so runner-style commands
(npx -> node -> server) leaked the real server process on every Windows
spawn test. taskkill /PID <pid> /T /F walks the descendant tree. Also
sets CREATE_NO_WINDOW on the spawned test process so the windowed exe
doesn't flash a console per test.

Closes #13

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

bcc_core.py

@@ -1581,6 +1581,23 @@ def server_log_path(name: str) -> Path | None:
 _STDERR_CAP = 4096  # bytes
 
 
+def _kill_process_tree_windows(pid: int) -> None:
+    """
+    Kill `pid` and its whole descendant tree via `taskkill /T /F` (issue #13).
+
+    Popen.kill() only terminates the direct child; runner-style commands
+    (npx → node → server, cmd → real process) leave the actual server alive,
+    leaking a process on every Windows spawn test. taskkill walks the tree.
+    """
+    flags = getattr(subprocess, "CREATE_NO_WINDOW", 0)  # no console flash from the GUI exe
+    with contextlib.suppress(OSError):
+        subprocess.run(
+            ["taskkill", "/PID", str(pid), "/T", "/F"],
+            capture_output=True,
+            creationflags=flags,
+        )
+
+
 def spawn_test(data: dict, timeout: float = 3.0) -> dict:
     """
     Attempt to start a stdio server and observe it for `timeout` seconds.
@@ -1675,6 +1692,10 @@ def _spawn_test_impl(data: dict, timeout: float) -> dict:
     )
     if os.name != "nt":
         popen_kwargs["start_new_session"] = True  # own process group → clean kill
+    else:
+        # The packaged app is windowed (console=False); without this every
+        # spawn test of a console server flashes a console window.
+        popen_kwargs["creationflags"] = getattr(subprocess, "CREATE_NO_WINDOW", 0)
 
     try:
         proc = subprocess.Popen(args_list, **popen_kwargs)
@@ -1696,7 +1717,7 @@ def _spawn_test_impl(data: dict, timeout: float) -> dict:
             if os.name != "nt":
                 os.killpg(os.getpgid(proc.pid), _signal.SIGKILL)
             else:
-                proc.kill()  # best-effort on Windows
+                _kill_process_tree_windows(proc.pid)
         except OSError:
             pass
         with contextlib.suppress(subprocess.TimeoutExpired):