[Copilot]

Packet, CobsCodec, and Fletcher16 were internal implementation details sitting at the src/ root alongside public headers. The existing src/protocol/internal/ nesting was also inverted — internal should be the top-level category.

Changes

• src/internal/codec/ — moved Packet, CobsCodec, and Fletcher16 (the only files with .cpp implementations) here
• src/internal/protocol/ — moved wire-format headers (AckMessage, HandshakeMessage, LogMessage, Packed) from src/protocol/internal/ here, flattening the inverted nesting
• Updated all #include paths in TinyLink.h, the moved headers, TinyProtocol.h comments, and all test files

Resulting layout

src/
├── TinyLink.h / TinyProtocol.h / version.h   ← public
├── protocol/                                  ← public, header-only
├── adapters/                                  ← public, header-only
└── internal/
    ├── codec/       ← header + cpp (CobsCodec, Fletcher16, Packet)
    └── protocol/    ← header-only wire formats (AckMessage, HandshakeMessage, LogMessage, Packed)

The internal/ boundary is now the single, top-level marker for implementation details, with codec/ vs protocol/ distinguishing the two internal concerns.