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>

minimateplus/client.py

@@ -1755,17 +1755,20 @@ def _decode_monitor_status(data: bytes) -> MonitorStatus:
     data is the raw S3 frame .data attribute (includes the 11-byte section
     header, so field offsets below are relative to data[11]).
 
-    Monitoring flag (confirmed 4-8-26/2ndtry, full byte diff analysis):
-      section[6] == 0x00 → idle
-      section[6] == 0x10 → monitoring
+    NOTE: frame.data has the checksum byte already stripped by S3FrameParser
+    (_finalise returns raw_payload[5:] where raw_payload = body[:-1]).
+    There is NO trailing checksum byte in section.
 
-    The payload size varies (52–55+ bytes) but the battery/memory block is
-    always the last 10 bytes before the trailing checksum byte:
+    Monitoring flag (confirmed 4-8-26/2ndtry, byte diff of all 144 data frames):
+      section[1] == 0x00 → idle
+      section[1] == 0x10 → monitoring
 
-      section[-11:-9]  battery × 100  uint16 BE  (0x02A8 = 6.80 V)
-      section[-9 :-5]  memory_total   uint32 BE  bytes
-      section[-5 :-1]  memory_free    uint32 BE  bytes
-      section[-1]      checksum       (not data)
+    The payload length varies (46–49 bytes) — IDLE is 46-47, MONITORING is 48-49.
+    The battery/memory block is always the last 10 bytes of section (no checksum):
+
+      section[-10:-8]  battery × 100  uint16 BE  (0x02A8 = 6.80 V)
+      section[-8 :-4]  memory_total   uint32 BE  bytes
+      section[-4:]     memory_free    uint32 BE  bytes
 
     Values confirmed from 4-8-26/2ndtry capture (BE11529):
       battery    0x02A8 = 680 → 6.80 V
@@ -1780,32 +1783,30 @@ def _decode_monitor_status(data: bytes) -> MonitorStatus:
         len(data), len(section), section.hex(),
     )
 
-    # Monitoring flag: section[6] (CORRECTED 2026-04-08 — was wrongly section[1]).
-    # Byte diff of 2ndtry BW-S3 captures confirms section[6] flips 0x00↔0x10
-    # exactly at the start/stop monitoring transitions (0xE3 frame #36 / #132).
-    is_monitoring = len(section) > 6 and section[6] == 0x10
+    # Monitoring flag: section[1] == 0x10.
+    # Confirmed from byte diff of all 144 0xE3 data frames in 4-8-26/2ndtry capture:
+    # section[1] = 0x00 in all IDLE frames, 0x10 in all MONITORING frames.
+    # (section[6] also changes but has non-binary values 0xea/0x07 — device-specific.)
+    is_monitoring = len(section) > 1 and section[1] == 0x10
 
     battery_v     = None
     memory_total  = None
     memory_free   = None
 
-    # Battery and memory offsets are RELATIVE TO THE END of the section.
-    # The payload length varies (52–55+ bytes) depending on monitoring state and
-    # internal counters, but the battery/memory block is always the last 10 bytes
-    # before the checksum (section[-1]).
+    # Battery and memory at relative-from-end offsets.
+    # Payload length varies (46–49 bytes) but the battery/memory block is always
+    # the last 10 bytes. No checksum byte — it was stripped by S3FrameParser.
     #
-    #   section[-11:-9]   battery  × 100   uint16 BE   0x02A8 = 6.80 V
-    #   section[-9 :-5]   memory_total     uint32 BE   ≈ 960 KB on BE11529
-    #   section[-5 :-1]   memory_free      uint32 BE   decreases as events fill
-    #   section[-1]       frame checksum   (not data)
+    #   section[-10:-8]   battery  × 100   uint16 BE   0x02A8 = 6.80 V
+    #   section[-8 :-4]   memory_total     uint32 BE   ≈ 960 KB on BE11529
+    #   section[-4:]      memory_free      uint32 BE   decreases as events fill
     #
-    # Confirmed stable across IDLE (52b), MONITORING (55b), and counter-jitter
-    # IDLE variants (53b) from 4-8-26/2ndtry full capture analysis.
-    if len(section) >= 11:
-        batt_raw     = struct.unpack(">H", section[-11:-9])[0]
+    # Confirmed stable across IDLE (46b), MONITORING (48-49b) variants.
+    if len(section) >= 10:
+        batt_raw     = struct.unpack(">H", section[-10:-8])[0]
         battery_v    = batt_raw / 100.0
-        memory_total = struct.unpack(">I", section[-9:-5])[0]
-        memory_free  = struct.unpack(">I", section[-5:-1])[0]
+        memory_total = struct.unpack(">I", section[-8:-4])[0]
+        memory_free  = struct.unpack(">I", section[-4:])[0]
 
     return MonitorStatus(
         is_monitoring=is_monitoring,