Refactor Logger to use PIMPL and add SetLevel

[Konstantysz]

Description

This PR refactors the Kappa::Logger to support setting log levels while completely hiding spdlog implementation details from the public API.

Key Changes:

• Hiding Implementation (PIMPL Pattern): Moved spdlog includes and the spdlog::logger member into src/Logger.cpp.
• Custom LogLevel Enum: Introduced Kappa::LogLevel to replace spdlog enums in the API.
• Added SetLevel: Support for dynamic log level adjustment.
• Unit Tests: Added ests/TestLogger.cpp to verify level switching and formatting.

Verified with
inja-debug preset and all tests passed.

[github-actions]

📊 Test Coverage Report

Coverage Statistics

Metric	Coverage
Lines	61.68%
Functions	58.76%