Here are the data structures with brief descriptions:

Caudio_sink_t	A PCM output backend (wired I2S DAC or Bluetooth A2DP)
Cautonomy_status_t	Snapshot of the running test, for the in-progress screen
Cbluetooth_device_t	One device found during a scan; the unit the UI lists and selects from
Cbt_known_device_t	The identity of a remembered device, enough to reconnect without scanning again
Cclock_cfg
Cctrl_xy_t
Cdecoder_backend_t	One format backend (MP3 or WAV). Single-stream: one open at a time
Cdecoder_format_t	PCM format of the open stream, read once after decoder_open()
Cfb_entry_t
Cfuel_gauge_data_t	Snapshot of the gauge's application-facing outputs
Cfuel_gauge_debug_t	Register-level snapshot for the debug page: config vs expected + learned state
Cinput_debounce_t	Debounce/edge state. Zero-initialise before the first call (memset or {0})
Cinput_diag_t	Live button snapshot for the diagnostics screen
Cmenu_item_t	A single menu row
Cmenu_screen_t	A scrollable list menu
Cmp3_frame_hdr_t	Decoded fields of one MPEG Layer III frame header
Cpipe_cmd_t
Cplayer_status_t	Snapshot of the transport for the UI
Cplaylist_track_t	The resolved current track, filled by the navigation/read calls
Cpower_state_t	Cached battery/charge snapshot, refreshed by power_tick()
Cscreen_t	A screen's behaviour, as a table of function pointers
Cscroll_t
Ctrack_meta_t	A track's tags and audio format, filled by track_meta_read()