fix(node): tell whether a pid is alive on Linux too

_pid_alive only ran tasklist, so request_graceful_stop raised
FileNotFoundError off Windows and two real-daemon tests failed on Linux.
Read /proc there, counting a zombie as gone.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

1 files changed, 8 insertions, 0 deletions

diff --git a/packages/meshbay-node/src/meshbay_node/platform.py b/packages/meshbay-node/src/meshbay_node/platform.py
index 09397a2..2255036 100644
--- a/packages/meshbay-node/src/meshbay_node/platform.py
+++ b/packages/meshbay-node/src/meshbay_node/platform.py
@@ -226,6 +226,14 @@ def _startup_vbs() -> Path:
 def _pid_alive(pid: int) -> bool:
     """Whether a process with this pid exists, in any session (tasklist lists
     session 0 too, where a service-mode daemon runs; opening it would not)."""
+    if sys.platform != "win32":
+        # A zombie counts as gone: it has exited, and only its parent has not
+        # reaped it yet -- os.kill(pid, 0) would still find it.
+        try:
+            stat = Path(f"/proc/{pid}/stat").read_text(encoding="ascii")
+        except OSError:
+            return False
+        return stat.rsplit(")", 1)[1].split()[0] != "Z"
     r = subprocess.run(["tasklist", "/FI", f"PID eq {pid}", "/NH", "/FO", "CSV"],
                        capture_output=True, text=True)
     return f'"{pid}"' in r.stdout