Here is a list of all files with brief descriptions:

core_types
gfx
elements.h	Header file defining basic structures for 2D graphic elements
numeric
datatypes.h	Enumeration definitions for various numerical data types
radix.h	Enumeration for numerical radix options
drivers
display
font
fonts
font_5x7_ascii.c	Source file containing the ASCII font table data
font.c	Source file containing character lookup function for text rendering
font.h	Header file for font handling and character bitmap retrieval
ssd130x
frame
frame.c	Source file implementing frame management and drawing functions for SSD130X display
frame.h	Header file for frame handling on SSD130X display
tty
tty.c	Source file with implementation of TTY text output interface on SSD130X display
tty.h	Header file for TTY interface - text output on SSD130X display
ssd130x.c	Source file with implementation of SSD130X display driver functions
ssd130x.h	Header file for SSD130X display control via hardware or software TWI/I2C
hal
avr0
twi
twi.c	Source file with implementation of hardware TWI/I2C functions and macros
twi.h	Header file with declarations and macros for hardware TWI/I2C
twi_soft
twi_soft.c	Source file with implementation of software TWI/I2C functions and macros
twi_soft.h	Header file with declarations and macros for software TWI/I2C
common
defines
TWI_defines.h	Macro definitions for TWI/I2C communication flags and operations
enums
TWI_enums.h	Enumerations for TWI/I2C interface modes, operations, acknowledgment, and error codes
utils
macros
stringify.h	Helper macros for stringification in C preprocessor
systick
systick.c	Implements the SysTick utility functions
systick.h	Header file with declarations and macros for systick utility