fix(lf): identify_lf fails clearly on a raw async device (not "coroutine has no len")

capture drives the device synchronously (as Proxmark3.sync()'s proxy exposes it). Passing a
bare async Proxmark3() — whose methods return un-awaited coroutines, and which isn't even
connected — blew up deep in the demod with "object of type 'coroutine' has no len()". Now
identify_lf detects a coroutine-function device up front and raises an actionable TypeError
pointing at Proxmark3.sync().

The mock-based tests returned plain values, so they never exercised the async method shape;
added a test using a real coroutine function that reproduces and guards it.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

pm3py/lf/capture.py

@@ -7,6 +7,8 @@ combines the two into "is it a T5577, and what is it (emulating)?".
 """
 from __future__ import annotations
 
+import inspect
+
 from . import protocols
 
 # how many envelope samples to pull from BigBuf after a capture (a T55xx/EM frame is a few
@@ -120,6 +122,15 @@ def identify_lf(device, emit=None) -> dict | None:
     if device is None:
         return None
 
+    # capture drives the device *synchronously* (as Proxmark3.sync()'s proxy exposes it). A raw
+    # async Proxmark3() returns un-awaited coroutines here — fail with an actionable message
+    # instead of a cryptic "coroutine has no len()" deep in the demod.
+    if inspect.iscoroutinefunction(getattr(getattr(device, "lf", None), "read", None)):
+        raise TypeError(
+            "identify_lf needs a synchronous device — use Proxmark3.sync(), not Proxmark3(). "
+            "(A bare Proxmark3() is the async API and isn't connected.)"
+        )
+
     # Force the device debug level to NONE for the duration: on debug-heavy firmware a raised
     # g_dbglevel makes LF acquisition emit Dbprintf frames over USB, which interleave with the
     # sample download and desync it. This is the device-side knob (not the C client's g_debugMode

tests/test_lf_demod.py

@@ -363,6 +363,20 @@ class TestIdentifyLF:
     def test_no_device(self):
         assert identify_lf(None) is None
 
+    def test_rejects_async_device(self):
+        # a raw async Proxmark3() exposes coroutine-function methods — must fail with a clear
+        # message pointing at Proxmark3.sync(), not blow up deep in the demod on an un-awaited
+        # coroutine (the MagicMock tests couldn't catch this since mocks return plain values)
+        dev = MagicMock()
+        async def _aread(*a, **k):
+            return {}
+        dev.lf.read = _aread
+        try:
+            identify_lf(dev)
+            assert False, "expected TypeError for an async device"
+        except TypeError as e:
+            assert "sync" in str(e).lower()
+
 
 class TestCaptureValidation:
     def test_looks_like_samples_rejects_memory(self):