fix: BATTERY handler drops frames shorter than 3 bytes (level field guard)

A BATTERY frame with len(data) < 3 caused dbuf.read(2) to return short
bytes; int.from_bytes(b"", ...) silently yielded 0, propagating a bogus
level=0 to HA sensors.  Same silent-zero class as N07 (storage fields).

Option B: early-return with debug log, matching the NEW-C pattern for
STATUS_RESPONSE.  No BATTERY event is dispatched for malformed frames.

Not in the original forensics report — discovered during G1 N07 work and
logged in issues_log.md.  Resolved here because no later branch touches
this handler.

Files changed:
- src/meshcore/reader.py: add `if len(data) < 3: return` guard before
  the level read in the BATTERY branch
- tests/unit/test_reader.py: add test_g1_battery_too_short_for_level —
  sends a 1-byte frame (type only), asserts no BATTERY event dispatched
  and debug log emitted

src/meshcore/reader.py

@@ -341,12 +341,20 @@ class MessageReader:
                 await self.dispatcher.dispatch(Event(EventType.CONTACT_URI, result))
 
             elif packet_type_value == PacketType.BATTERY.value:
+                # Full RESP_CODE_BATT_AND_STORAGE: 1 type + 2 level + 4 used_kb + 4 total_kb = 11 bytes.
+                # Minimum viable frame is 3 bytes (type + level).  Shorter frames are
+                # malformed — dbuf.read(2) would return short bytes and
+                # int.from_bytes(b"", ...) silently yields 0 (same class as N07).
+                if len(data) < 3:
+                    logger.debug(
+                        "BATTERY frame too short for level field "
+                        f"({len(data)} bytes < 3), skipping"
+                    )
+                    return
                 battery_level = int.from_bytes(dbuf.read(2), byteorder="little")
                 result = {"level": battery_level}
-                # Full RESP_CODE_BATT_AND_STORAGE frame is 11 bytes:
-                # 1 type + 2 level + 4 used_kb + 4 total_kb. The previous
-                # `len(data) > 3` guard let 4-10 byte truncated frames through,
-                # producing silent zero values for used_kb/total_kb because
+                # The previous `len(data) > 3` guard let 4-10 byte truncated frames
+                # through, producing silent zero values for used_kb/total_kb because
                 # io.BytesIO.read() returns short data without raising.
                 if len(data) >= 11:  # has storage info as well
                     result["used_kb"] = int.from_bytes(dbuf.read(4), byteorder="little")