refactor: extract shared message protocol for runner and launcher

[AnemoFlower]

This pull request introduces a new shared message protocol library to standardize communication between the Runner and Launcher executables, and refactors both to use this protocol for inter-process messaging over Unix Domain Sockets. The changes improve code maintainability, robustness, and clarity in the message exchange mechanism.

Protocol and Communication Refactoring:

• Introduced a new MessageProtocol library target, which defines a unified Message enum and encoding/decoding logic for inter-process communication. Both Runner and Launcher now depend on this library. (Package.swift, Sources/MessageProtocol/Protocol.swift, Sources/MessageProtocol/ByteReader.swift) [1] [2] [3]
• Refactored the Runner and Launcher to use the new MessageProtocol for all socket communication, replacing custom byte handling with structured message encoding/decoding. (Sources/Runner/main.swift, Sources/Launcher/main.swift) [1] [2] [3] [4] [5] [6]

Code Simplification and Robustness:

• Simplified socket reading and writing logic by introducing the ByteReader and SocketReader abstractions, improving error handling and code clarity. (Sources/MessageProtocol/ByteReader.swift, Sources/Launcher/main.swift) [1] [2]
• Improved error handling and logging for unexpected or malformed messages, and made message handling more robust to incomplete or unknown message types. (Sources/MessageProtocol/Protocol.swift, Sources/Launcher/main.swift) [1] [2]

Minor Cleanups:

• Minor code cleanups and consistency improvements in argument construction and process launching. (Sources/Launcher/main.swift) [1] [2]