Add configurable C++ logger and forward logs to Unity Console

[Gogoshika-ga]

Add a logger hook in the C++ SDK and use it in Unity to surface native logs.

Why

C++ logs are hard to access from Unity during debugging.

Scope

• Add settable logger callback/interface in C++ SDK.
• Register a Unity logger bridge.
• Map levels to Debug.Log, Debug.LogWarning, Debug.LogError.
• Keep forwarding configurable (off/basic/verbose).

Acceptance Criteria

• C++ SDK supports custom logger injection.
• Unity receives and prints C++ logs with correct levels.
• Logging can be enabled/configured.
• No regression when logger is not set.
• Docs updated.