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_hf.py

@@ -16,3 +16,29 @@ def test_hf_tune():
     # Firmware requires the START(1)/MEASURE(2)/STOP(3) mode-byte sequence
     modes = [call.args[1] for call in t.send_ng.call_args_list]
     assert modes == [bytes([1]), bytes([2]), bytes([3])]
+
+
+def test_hf_dropfield():
+    t = AsyncMock()
+    hf = HFCommands(t)
+    result = asyncio.get_event_loop().run_until_complete(hf.dropfield())
+    assert result["status"] == 0
+    # Firmware CMD_HF_DROPFIELD sends no reply -> must be fire-and-forget
+    t.send_ng_no_response.assert_awaited_once()
+    assert t.send_ng_no_response.call_args.args[0] == Cmd.HF_DROPFIELD
+    t.send_ng.assert_not_called()
+
+
+def test_hf_sniff():
+    t = AsyncMock()
+    hf = HFCommands(t)
+    # Firmware replies with a uint16 sample count (1234 = 0x04D2)
+    t.send_ng.return_value = PM3Response(cmd=Cmd.HF_SNIFF, status=0,
+                                         reason=0, ng=True, data=b"\xd2\x04")
+    result = asyncio.get_event_loop().run_until_complete(
+        hf.sniff(samples_to_skip=5, triggers_to_skip=6, skip_mode=1, skip_ratio=2))
+    assert result["samples"] == 1234
+    assert result["status"] == 0
+    # Always sends the full 10-byte PACKED {u32 u32 u8 u8} struct
+    payload = t.send_ng.call_args.args[1]
+    assert payload == b"\x05\x00\x00\x00\x06\x00\x00\x00\x01\x02"