Display controls (enum/get/set):
- Add PROTO_DISPLAY_CTRL_SCALE/ANCHOR/NO_SIGNAL_FPS constants to protocol.h
- handle_enum_controls: if device index maps to an active display slot,
return the three display controls (scale, anchor, no_signal_fps)
- handle_get_control: read display control values from slot under mutex
- handle_set_control: write display control values to slot under mutex;
scale/anchor are applied to the viewer by display_loop_tick each tick
Device IDs in enum-devices output:
- Proto_Display_Device_Info gains device_id field (wire format +2 bytes)
- handle_enum_devices computes device_id = total_v4l2 + display_index
- on_video_node/on_standalone callbacks take int* userdata to print [idx]
- on_display prints [device_id] from the wire field
Bug fix — protocol error on invalid device index:
- proto_read_enum_controls_response: early-return APP_OK after reading
status if status != OK; error responses have no count/data fields, so
the CUR_CHECK on count was failing with "payload too short"
Helpers added to main.c:
- count_v4l2_devices(): sum of media vnodes + standalone
- find_display_by_device_idx(): maps flat index to Display_Slot
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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