[CORE] Implement logging system with spdlog

[mrgreenapple24]

Goal

Structured, levelled logging for both engine-internal and client code, compileable to no-ops in distribution builds.

Tasks

• Create Log class initialising two spdlog::logger instances: Core and Client
• Add console sink (colored) and rotating file sink
• Define macros:
  • ELYSIUM_CORE_TRACE/INFO/WARN/ERROR/CRITICAL
  • ELYSIUM_TRACE/INFO/WARN/ERROR/CRITICAL (client)
• Strip all macros to no-ops when ELYSIUM_DIST is defined
• Call Log::Init() as the first thing in the entry point

Acceptance Criteria

Engine and client code can log at all levels. File Elysium.log is written. Zero log output in DIST build.