feat: add high-water mark state tracking to ach_server + fix monitoring flag

ach_server.py:
- Add ach_state.json per-unit state tracking (keyed by serial number)
- count_events() before any download; skip session if no new events since last call-home
- Download only events beyond the previous high-water mark (all_events[last_count:])
- --max-events N safety cap for first-run units with many stored events
- state_path and max_events wired through AchSession constructor and serve()

client.py (_decode_monitor_status):
- Revert monitoring flag to section[1] == 0x10 (was incorrectly changed to section[6])
- Fix battery/memory offsets to section[-10:-8], [-8:-4], [-4:] (no trailing checksum byte)
- Both confirmed by full byte diff of all 144 0xE3 data frames in 4-8-26/2ndtry capture

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

.gitignore

@@ -1,28 +1,28 @@
-/bridges/captures/
-/example-events/
-
-/manuals/
-
-# Python bytecode
-__pycache__/
-*.py[cod]
-
-# Virtual environments
-.venv/
-venv/
-env/
-
-# Editor / OS
-.vscode/
-*.swp
-.DS_Store
-Thumbs.db
-
-# Analyzer outputs
-*.report
-claude_export_*.md
-
-# Frame database
-*.db
-*.db-wal
-*.db-shm
+/bridges/captures/
+/example-events/
+
+/manuals/
+
+# Python bytecode
+__pycache__/
+*.py[cod]
+
+# Virtual environments
+.venv/
+venv/
+env/
+
+# Editor / OS
+.vscode/
+*.swp
+.DS_Store
+Thumbs.db
+
+# Analyzer outputs
+*.report
+claude_export_*.md
+
+# Frame database
+*.db
+*.db-wal
+*.db-shm