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

tests/unit/test_reader.py

@@ -162,6 +162,33 @@ async def test_g1_battery_short_frame_omits_storage_fields():
     )
 
 
+@pytest.mark.asyncio
+async def test_g1_battery_too_short_for_level(caplog):
+    """BATTERY frame shorter than 3 bytes must be dropped entirely (Option B).
+
+    A 1-byte frame (just the packet-type byte 0x0c, no level bytes) would cause
+    dbuf.read(2) to return b"" and int.from_bytes(b"", ...) to silently yield 0.
+    The fix adds an early return with a debug log, matching the NEW-C pattern.
+    """
+    dispatcher = _CapturingDispatcher()
+    reader = MessageReader(dispatcher)
+
+    # 1-byte BATTERY frame: only the type byte, no level payload.
+    too_short = bytearray.fromhex("0c")
+
+    with caplog.at_level(logging.DEBUG, logger="meshcore"):
+        await reader.handle_rx(too_short)
+
+    battery_events = [e for e in dispatcher.events if e.type == EventType.BATTERY]
+    assert len(battery_events) == 0, (
+        "BATTERY frame shorter than 3 bytes must not dispatch an event"
+    )
+    debug_records = [
+        r for r in caplog.records if "BATTERY frame too short" in r.message
+    ]
+    assert debug_records, "Expected a debug log about the short BATTERY frame"
+
+
 @pytest.mark.asyncio
 async def test_g1_status_response_short_frame_skipped(caplog):
     """G1/NEW-C: short STATUS_RESPONSE push frame must be skipped, not parsed with bogus zeros."""