Device UAC is speaker-only: no microphone / capture endpoint #7

What is missing

The device-side UAC function presents a stereo speaker with feedback (CFG_TUD_AUDIO_ENABLE_EP_OUT, USBIF_AUDIO_SPEAKER_STEREO_FB_DESCRIPTOR). There is no isochronous IN endpoint, so a board cannot present as a USB microphone or a full-duplex sound card.

Why it matters

Host-side capture works (host_uac_read, usbif.uac_audio.input, proven with a commercial USB mic). The reverse pairing — an S3 or a PC capturing from a PyDevices board — cannot exist until the device side grows a mic stream. The examples catalog therefore has usb_mic.py (host capture) but no device-side capture counterpart.

Evidence

• src/usbif_tusb_ext.h: CFG_TUD_AUDIO_ENABLE_EP_OUT (1) only; speaker descriptor only
• No CFG_TUD_AUDIO_ENABLE_EP_IN / TX path in the module

Not claiming

Host UAC capture is fine. This issue is device-role mic only.