Consolidate logging output behind spdlog backend

[VolkerChristian]

Motivation

• SNode.C already uses spdlog, but Logger.cpp mixed backend state and sink setup with the semantic and legacy logging facade making the implementation hard to maintain.
• Introduce a clear backend boundary so spdlog owns how/where already-formatted lines are emitted while SNode.C keeps owning semantic policy, formatting and public API.

Description

• Added an internal backend abstraction logger::detail::SpdlogBackend (header + implementation) and moved stdout/file sinks, rotating file setup, tick formatting, quiet/color state, and backend logger ownership into it (src/log/detail/SpdlogBackend.h, src/log/detail/SpdlogBackend.cpp).
• Simplified Logger.cpp into a stable facade that delegates initialization, tick resolver, quiet/color toggles, file enable/disable, legacy emission and level/verbose checks to the backend while preserving LogManager/LogRecord semantics and public APIs.
• Preserved semantic behavior by keeping LogManager::shouldEmit() and LogManager::formatRecord() in the semantic layer and passing the already-formatted semantic line to the backend for emission.
• Updated src/log/CMakeLists.txt to compile and install the new backend source/header and retained synchronous emission (no async mode introduced).

Testing

• Ran cmake -S . -B cmake-build which configured the project successfully.
• Ran cmake --build cmake-build --parallel 2 which built the project successfully.
• Ran ctest --test-dir cmake-build -R "Log|Semantic" --output-on-failure which executed and reported No tests were found!!! for the focused logging regex.
• Ran ctest --test-dir cmake-build --output-on-failure which executed and reported No tests were found!!! for the full test run in this build tree.
• Ran git diff --check to validate whitespace and patch issues and it returned clean.
• Ran clang-format -i src/log/Logger.cpp src/log/detail/SpdlogBackend.cpp src/log/detail/SpdlogBackend.h to format touched files.

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Codex Task