docs: Add Doxygen comments to library files

Added Doxygen-style comments to all C source and header files in the
 and  directories. This improves
code documentation and enables VSCode tooltip help.

Additionally, short inline comments were added to all global variables
for better clarity.

software/include/lib/fwu.h

@@ -3,8 +3,45 @@
 
 #include <stdint.h>
 
+/**
+ * @file fwu.h
+ * @brief API for the Firmware Update (FWU) library.
+ *
+ * This library provides the core logic for handling the over-the-air firmware
+ * update process via Modbus. It manages the data buffer, processes commands,
+ * and calculates CRC checksums for data verification.
+ */
+
+/**
+ * @brief Initializes the firmware update module.
+ *
+ * This function currently does nothing but is a placeholder for future
+ * initialization logic.
+ */
 void fwu_init(void);
+
+/**
+ * @brief Handles incoming Modbus register writes related to firmware updates.
+ *
+ * This function is the main entry point for the FWU process. It parses the
+ * address and value from a Modbus write operation and takes appropriate action,
+ * such as storing metadata (offset, size) or data chunks, and processing
+ * commands (verify, finalize).
+ *
+ * @param addr The Modbus register address being written to.
+ * @param reg The 16-bit value being written to the register.
+ */
 void fwu_handler(uint16_t addr, uint16_t reg);
+
+/**
+ * @brief Gets the CRC16-CCITT of the last received firmware chunk.
+ *
+ * After a data chunk is fully received into the buffer, this function can be
+ * called to retrieve the calculated CRC checksum. The master can then compare
+ * this with its own calculated CRC to verify data integrity.
+ *
+ * @return The 16-bit CRC of the last chunk.
+ */
 uint16_t fwu_get_last_chunk_crc(void);
 
 #endif // FWU_H