feat(mgmt): Ping both NICs at once, and stop counting ARP as loss

The bench run reported FAIL=3 with no NIC errors and no drops. All three
failures were on eth1 and all three were missing exactly icmp_seq=1,
with the other nine replies present and sub-millisecond. That is
neighbour resolution: once the ARP entry for the target expires, the
first echo request is spent resolving it and ping counts it as loss.
Rounds sat about 24 s apart, right on the edge of the default reachable
time, so it happened on some rounds and not others.

mgmt_ping_monitor.sh V3.0.0:

- One discarded ping per NIC before the measured run resolves ARP, so
  MAX_LOSS_PCT can stay at 0 and a FAIL means a real drop. Raising the
  threshold instead would have hidden genuine single-packet loss, and a
  longer settle would not help -- nothing triggers ARP until the ping.
- Both NICs now ping at the same time and keep accumulating until
  stopped, rather than alternating fixed bursts. Each writes its own raw
  capture.
- `stop` sends SIGINT rather than SIGTERM, because ping prints its
  statistics block on interrupt and that block is what the report
  parses, then renders into the log: the last TAIL_LINES entries per
  NIC, each one's statistics and verdict, and `ip -s link show` for both
  interfaces.
- `-D -O` are on by default so every line carries a timestamp and a
  request that got no reply is visible in the tail instead of merely
  absent. Clear PING_EXTRA_OPTS if the platform's ping rejects them.

Since the script no longer re-execs itself to daemonise, `bash
mgmt_ping_monitor.sh start` works without the execute bit.

.gitignore: the new *.raw / *.pid / *.state runtime files were outside
the *.log rule and would have been committed.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014RetWKFZFG1ZHcitQAyhwM

.gitignore

@@ -12,6 +12,11 @@ For_AI/
 !**/logs/.gitkeep
 !**/Logs/.gitkeep
 
+# monitor 腳本的執行期產物（原始擷取、pid、狀態）
+*.raw
+*.pid
+*.state
+
 # 打包輸出：產出不進 git，打包工具本身要進
 publish/*
 !publish/.gitkeep