refactor: make codec layer fully header-only

[Copilot]

The three codec files (Fletcher16, CobsCodec, Packet) were split into .h/.cpp pairs while everything else — the main TinyLink engine, all platform adapters, and all protocol types — was header-only. This inconsistency required a separate compilation step for no real benefit.

Changes

• Merged .cpp bodies into headers — function definitions moved inline, .cpp files deleted
• static inline preserved on packet_size; inline applied to all other codec functions
• Packet.h gains #include <string.h> and #include "Fletcher16.h" (previously pulled in transitively via the .cpp)

// Before: declaration only in header, body in .cpp
uint16_t fletcher16(const uint8_t* data, size_t len);

// After: fully inline in header
inline uint16_t fletcher16(const uint8_t* data, size_t len) { ... }

With a single MCU translation unit this removes call overhead on the checksum and COBS hot paths, and the build no longer needs to compile or link separate codec objects.