fix: correct event count field offset and eliminate count_events() walk

_decode_event_count: read uint16 BE at offset 10 (confirmed 2026-04-10 from
live BE11529 event index — data[10:12]=0x0006=6, matches device LCD).
Previous uint32 at offset 3 always returned 1 regardless of event count.

ach_server.py: use device_info.event_count (already fetched during connect())
instead of calling count_events() separately. This saves 2*N round-trips and
avoids the 1F linked-list walk which was overcounting on some devices.
count_events() kept as fallback when connect() is skipped (--events-only).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

bridges/ach_server.py

@@ -216,13 +216,20 @@ class AchSession:
             unit_key = serial or self.peer  # fall back to IP if no serial
             last_count = state.get(unit_key, {}).get("event_count", 0)
 
-            try:
-                current_count = client.count_events()
-                log.info("  Unit has %d stored event(s); last downloaded count: %d",
-                         current_count, last_count)
-            except Exception as exc:
-                log.error("  [FAIL] count_events failed: %s", exc)
-                return
+            # Use the event count already read from the event index during connect().
+            # This is fast (no extra round-trips) and confirmed accurate (matches LCD).
+            # Falls back to count_events() only if connect() wasn't called.
+            if device_info is not None:
+                current_count = device_info.event_count
+            else:
+                try:
+                    current_count = client.count_events()
+                except Exception as exc:
+                    log.error("  [FAIL] count_events failed: %s", exc)
+                    return
+
+            log.info("  Unit has %d stored event(s); last downloaded count: %d",
+                     current_count, last_count)
 
             if current_count <= last_count:
                 log.info("  [OK] No new events since last call-home -- nothing to download")