fix(hf,hw): fire-and-forget dropfield, full HF_SNIFF struct, standalone payload

dropfield waited on a reply the firmware never sends (bare hf_field_off();break;) -> 2s stall + raise; use send_ng_no_response. hf.sniff under-sent the 10-byte {u32,u32,u8,u8} struct and mis-parsed the uint16 reply. hw.standalone sent an empty payload for a {arg,mlen,mode[10]} struct and blocked; pack it + fire-and-forget. Pre-existing, independent of rebase.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_hw.py

@@ -99,3 +99,17 @@ def test_led_pwm_non_capable_raises():
     with pytest.raises(PM3Error, match="do not support"):
         asyncio.get_event_loop().run_until_complete(
             hw.led("blue", brightness=50))  # blue=D, not PWM on Easy
+
+
+def test_standalone():
+    transport = make_mock_transport()
+    hw = HWCommands(transport)
+    result = asyncio.get_event_loop().run_until_complete(hw.standalone(arg=3, mode="14a"))
+    assert result["status"] == 0
+    # Firmware RunMod() never replies -> fire-and-forget
+    transport.send_ng_no_response.assert_awaited_once()
+    cmd, payload = transport.send_ng_no_response.call_args.args
+    assert cmd == Cmd.STANDALONE
+    # PACKED {uint8 arg; uint8 mlen; uint8 mode[10]} = 12 bytes
+    assert payload == struct.pack("<BB", 3, 3) + b"14a" + bytes(7)
+    transport.send_ng.assert_not_called()