A Bluetooth conference mic on Windows connects over the hands-free profile,
which downsamples to narrowband and drops frames. faster-whisper then returns
garbled text. The same mic over USB Audio Class transcribes cleanly, so this is
the transport, not the model.

What made it cost an afternoon is that wake detection does not degrade with it.
A live "computah ..." still scores 0.998, because the wake model matches a short
fixed pattern that survives the damage while a full sentence does not. The loop
looks healthy right up to the transcript, so the mic is the last thing suspected.
Hence a warning at startup rather than only a README note.

Two signals: a hands-free marker in the device name, and a native rate below the
pipeline's 16 kHz. The rate read is the device's advertised rate, not the one the
stream opened at -- _open() asks for 16 kHz first and WASAPI auto_convert grants
it even on a narrowband device, so the opened rate reports 16000 while the audio
is already ruined. Checking that field would have detected nothing.

The classifier is its own module rather than part of audio.py because audio.py
imports sounddevice, which loads native PortAudio and is absent on plenty of
boxes. Judging a device's advertised properties needs no hardware, so keeping it
separate makes it testable everywhere, matching the hardware-free-core split the
project already draws.

Also surfaced where someone choosing a mic will look: --list tags unsuitable
inputs, and --test-mic now exits 3 instead of reporting a clean pass on a device
it just warned about. A diagnostic that returns success on hardware it told you
not to use is how this stayed confusing.

Known blind spot, documented rather than papered over: wideband HFP negotiates
16 kHz and clears the rate check, and on Linux the name marker is usually absent,
so a clean result means "nothing detected", not "device is fine".

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The rate check read default_samplerate and then asserted the device 'is
narrowband' and told the operator to use a different mic. That field is what
the device advertises, not a measured capability: under WASAPI it is the
shared-mode capture format and the limit is real, but under ALSA or CoreAudio
it can be a default the device exceeds, so the flat claim could send someone
to replace a working mic.

Reports the advertised figure and what it does and does not establish per
host, and softens --test-mic's RESULT line and exit-3 help to read as the
classifier's verdict rather than as ground truth. The hands-free branch keeps
its certainty, since a name match identifies the device outright.

Kept consulting the advertised rate rather than the opened one: a successful
open at 16 kHz is what auto_convert fakes, which is the bug this feature
exists to catch. Separating an advertised default from a hard limit needs a
spectral check on real audio, and that costs a capture at startup.

Detection unchanged, so the 23 checks still pass.

The name and rate checks only see what a device chooses to report, which is
enough on Windows and not enough anywhere else. A bluez/PipeWire HFP source on
Linux exposes no hands-free marker and PipeWire resamples to 16 kHz and reports
16 kHz, so both existing signals read clean while the audio underneath came off
an 8 kHz transport. That hole was documented in the module and left open.

--test-mic already captures a few seconds, so judging that audio costs nothing
extra. Upsampling cannot invent content it never received, so an 8 kHz source
leaves the band above 4 kHz empty and the cliff at that edge is the evidence.

Measuring the cliff rather than total high-band energy is the part that took
work. Speech falls off steeply on its own, so a raw high-band fraction mostly
measures the tilt: on these signals a dull voice reads 0.00007% and a sloppily
resampled 8 kHz signal reads 0.005%, putting the genuine case on the wrong side
of the narrowband one. Comparing the 1 kHz band above the edge against the 1 kHz
below cancels the tilt and separates them by an order of magnitude in the right
order.

The threshold is measured rather than guessed, and _cliff_ratio is split out from
the verdict so a test can assert on the margin instead of only the verdict. That
test earned its place immediately: the first threshold sat under 2x from the
narrowband ceiling and had to move.

Refs #34.

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