Fix critical lock-expiry window found in Codex second pass

- poller.py: start lock_refresher() IMMEDIATELY after acquiring the lock,
  before the startup jitter and restart-storm deferral. Those sleeps can
  together exceed LOCK_TTL (restart defer is up to POLL_SWEEP_INTERVAL), so
  with the refresher started afterward the lock could expire mid-deferral and
  a second poller could begin sweeping concurrently — the exact hardware-storm
  risk this design prevents.
- hwgate.py: clamp POLL_CONCURRENCY to 1 unless POLL_ALLOW_UNSAFE_CONCURRENCY
  is set (fragile SAS hardware; >1 can drop expanders). Warn either way.
- build.sh: keep backward-compatible — a non-target first arg is treated as
  the commit message (./build.sh "msg" still works), known targets select
  web|poller|all.

poller.py

@@ -165,6 +165,13 @@ async def main() -> None:
         await asyncio.sleep(LOCK_TTL // 2)
     logger.info("poller lock acquired (%s)", TOKEN)
 
+    # Start the refresher IMMEDIATELY — before any pre-sweep sleep. The jitter
+    # and restart-storm deferral below can together exceed LOCK_TTL, so without
+    # an active refresher the lock would expire and a second poller could begin
+    # sweeping concurrently (a hardware-storm risk).
+    refresher = asyncio.create_task(lock_refresher())
+    refresher.add_done_callback(_critical_task_done)
+
     if not smartctl_available():
         logger.warning("smartctl not found — install smartmontools")
     if not sg_ses_available():
@@ -188,8 +195,6 @@ async def main() -> None:
             logger.info("last sweep %.0fs ago — deferring first sweep %.0fs", since, wait)
             await asyncio.sleep(wait)
 
-    refresher = asyncio.create_task(lock_refresher())
-    refresher.add_done_callback(_critical_task_done)
     leds = asyncio.create_task(led_consumer())
     try:
         while True: